The author of the present study prefaces his essay with a few general considerations upon the advances made in meteorology, especially in the study of the mechanics of the atmosphere, which is now coming to the front. Sailors will be the first to benefit by this movement of opinion; meteorology has invariably awakened in them a powerful interest, and this is only natural. "When one spends half of a lifetime upon the deck of a ship continually moving about, one meets the same phenomenon under so many different aspects that he cannot help asking himself what the determinative principle is." (Admiral Mottez.) The writer further states that his efforts have been directed to establishing upon a firm basis the conclusions that terminate his work and form its quintessence, so to speak.
NATURE OF THE AIR—THE ATMOSPHERE.
The terrestrial globe is surrounded by an envelope of air nearly inalterable in its composition at all points and altitudes: 79.20 per cent of azote and 20.80 of oxygen in volume. To those two gases, which are the principal constituent elements of our atmosphere, must be added aerial dust, carbonic acid, and vapor of water, which enter into the mixture in proportions varying according to place, season, and even the hour of the day.
The density of the air also varies with the spot under consideration, the temperature, the fraction of saturation, and, above all, the altitude. As we rise in the atmosphere, this density decreases in a rapid proportion that would assign to the aerial envelope a very near boundary, if close observations did not invalidate its accuracy beyond a certain limit. The measurements made by Bravais upon the aurora borealis permit us to reckon at 200,000 meters the height of the atmospheric column; some writers calculate it as high as 300,000 m.
Pressures.—This immense air-column produces upon every part of the earth a pressure whose mean annual value varies with the point under consideration.
The following table, arranged by Maury and reproduced in the nautical meteorology of Ploix and Caspari, gives the mean barometric height at sea in function of the latitude. We have selected in preference the southern hemisphere, owing to its vast preponderance of sea surface.
Latitudes | Barometer | No. of Observations |
From 0° to 5° | 760.46 | 3692 |
From 5° to 10° | 761.48 | 3924 |
From 10° to 15° | 762.70 | 4156 |
From 15° to 20° | 763.51 | 4248 |
From 20° to 25° | 764.59 | 4536 |
From 25° to 30° | 764.40 | 4780 |
From 30° to 36° | 763.30 | 6970 |
From 40° to 43° | 761.69 | 1703 |
From 43° to 45° | 756.40 | 1130 |
From 45° to 48° | 752.59 | 1174 |
From 48° to 50° | 752.33 | 672 |
From 50° to 53° | 748.78 | 665 |
From 53° to 55° | 745.73 | 475 |
From 56° to 60° | 743.95 | 1126 |
The annexed curve is a graphic representation of Maury’s table; it shows us the existence of a barometric maximum at the tropic and the steep gradients of pressure towards the 42d parallel.
Farther on we shall examine this curve in detail, but we can already perceive that there exists a palpable relation between the latitude and the corresponding barometric height.
What forces are held in equilibrium by a column of mercury of Torricelli? Are they simply the weight of the air and the tension of the vapor of water it contains, or rather the sum of molecular and electric attractions of the atmosphere?
It does not seem admissible, given the great thickness attributed to the aerial envelope, that beyond the limits of this same envelope the super-elevated layers are connected with the earth by means of their mer6 weight. At that height, the extremely rarefied and imponderable air has not ceased to be a gas and to possess all the properties of gas; it must therefore be endowed with an indefinite expansive energy. Thus we cannot conceive of the sudden boundary between a gaseous state, of whatever kind, and an absolute vacuum, except through the adjunction of a force independent of the density of the gas under consideration. This force, which acts as a barrier to our atmosphere and prevents it from disappearing little by little into the interplanetary spaces, is found in the electrical tension of the air, which by actual experiment has been proven to increase progressively in inverse ratio of its density. In that case, electrical attraction constitutes an element in the total barometric pressure, whose principal factors are the molecular weight of the air and the tension of the vapor of water it contains.
Should a distinction be made between the total pressure of air in apparent state of repose and the same pressure when the atmosphere is in motion? We do not think so. Air pressure, with the exception of well characterized cyclonic disturbances, hardly varies a few millimeters above or below the static pressure, according to whether the winds predominating on the surface are cold and dry, or warm and moist. We cannot see, on the other hand, why air moving in a horizontal direction should prevent the atmosphere from exercising all its weight upon the earth's surface, or be capable of developing internal dynamic action sufficient to interfere with gravitation. Many cases are cited where in strong northwest gales in the Mediterranean the barometer stood above the mean. The force of the wind seems, therefore, to have little influence upon the barometric height. It is not so, however, when the wind acts in a direction deviating from the horizontal; it is evident, a priori, that every downward vertical component causes a rise in the barometer, and every upward ascending motion of the air decreases the pressure.
RELATIONS OF BAROMETRIC PRESSURE TO ATMOSPHERIC MOTION.
1st Principle.—It is generally admitted that the differences of pressures are the primary causes of the movements of the air. Researches into those differences, combined with the examination of the rotation of the earth, constitute the study of atmospheric disturbances: as water is set in motion by the differences of levels, and no electric current can exist without potential differences, in the same way, also, air is not displaced nor can there be any atmospheric current except by virtue of a difference of pressure.
Sometimes the above principle is thus interpreted: Every wind has its motive in front of it. That interpretation is not a correct one, for very frequently winds move athwart their determinative causes. It would be more correct to say: To every atmospheric displacement corresponds a difference of pressure that has caused the movement. We shall see in the course of this study what share the active forces that the globe in motion distributes unequally over the different parts of its gaseous envelope, contribute to the aerial circulation.
2d Principle.—The second principle upon which we shall rely is the following: When a depression takes place at a given point, the air is attracted on every side with an energy proportionate to its expansive force; that is, to its density, at least in the generality of cases. From this proposition, which to us appears evident, we can evolve a very important law: Density is greater at the surface; it is therefore by changes of air surface, in the same manner as liquid in two vases communicating at the bottom is leveled, that the equilibrium of pressure will be sought, the points of lesser resistance at the surface acting as adducing channels. The equilibrium would be thus re-established, but the rotation of the earth modifies the direction of the air currents, and for this reason the equalization of pressures is never reached directly.
INFLUENCE OF ELECTRICITY ON THE MOTIONS OF THE ATMOSPHERE.
There is another natural force that may, we think, cause atmospheric disturbances, but only in a limited zone : this is electricity. It has been seen what presumed part it plays in the barometric pressure, and we may infer:
1. That a perceptible loss in the electric tension of the air at any given point is apt to determine a partial depression, hence a disturbance.
2. That an inrush from the upper strata into our immediate atmosphere (whether the inrush be direct or caused by the clouds that those elevated strata contained) must produce well characterized electrical phenomena, besides considerably lowering the temperature and thus causing snow and hail.
In reality the role of electricity in the atmosphere and its influence upon the displacements of the air are almost unknown.
It appears probable, a priori, that two neighboring clouds charged at different potentials are attracted one towards the other in order to equalize their difference, and that this attraction must bring on a displacement in the surrounding atmosphere. But can this effect be felt at great distances, knowing that electric attraction varies in inverse ratio to the square of the distance? We do not think so. The primary condition for a force to make a perceptible impression upon atmospheric inertia is the duration of this same force. Now, clouds cannot long keep electricity at a high potential; if the discharge is not immediate, the expansion goes on incessantly by means of the moist air, which is relatively a good conductor.
The source of electrical production is at the earth's surface, and the reservoir, in the upper regions. Everybody is familiar with the experiment with the Armstrong boiler. The boiler remains charged with negative electricity, while the wet steam that escapes from it is charged at a positive potential.
When water is evaporated on the surface of the globe by the sun's rays, the earth acts as the boiler, and the vapor of water, rising in the atmosphere and condensing, imparts by degrees to the surrounding air the electricity with which it is charged.
To this primary cause of the production of atmospheric electricity must be added another hypothetical one, the friction of the air against the earth's surface; a part of the work thus expended could be found in the upper regions under the form of electrical potential. It is certain that the upper strata of the atmosphere cannot go on receiving the electricity rising from the earth without expansion either by a sudden, tumultuous disruption, or slowly and by degrees; in other words, this accumulated force must find its use. Storms are one of the means of bringing on its return to zero. Is electricity the determinative principle, or else does it simply make use of the cirrus that rise to the upper regions to come down again in contact with the earth?
That is a hard problem to solve, considering the actual state of meteorologic knowledge.
Whatever it may be, and until proof of the contrary is brought forth, we shall continue to think that electrical attractions are the consequences, and not the causes of the motions of the air.
VAPOR OF WATER IN THE ATMOSPHERE.
Vapor of water plays a considerable part in the equilibrium of pressures. In order to justify its importance, it will suffice to mention the formula which expresses the weight P of a. volume of air V, at the temperature t, when the exterior pressure is H and the tension of the vapor of water, f.
The formula may be translated as follows:
With even pressures and temperatures, a liter of dry air is heavier than a liter of humid air.
If, therefore, the barometric pressure be the same in two neighboring countries, the one dry and the other humid, everything being otherwise equal, it is safe to say that the air column is less in the first than in the second. Inversely, if the heights be the same, it is because the level of the mercury is higher in the dry region. The tension of the vapor of water forms an integral part of the atmospheric pressure; being absolutely independent of the particular pressure of the air, it varies in a general manner with the temperature, and for a given center, with the surface of evaporation, the emitting power of the soil, and the activity of the strata that contain it.
For a fixed temperature, the expansive force of water vapor attains a maximum that it cannot exceed. It is then clear that as long as this force remains below its limit value, the changes it may undergo react integrally upon the barometric pressure.
When that maximum is reached, if the temperature remains unaltered, the tension of the vapor of water, and consequently the total pressure of the air, remains unaltered.
We have seen that the vapor of water is the means of electricity accumulating in the upper regions of the atmosphere ; it is also the vehicle of solar heat, which without it could not rise to any great height. It is a well known fact that luminous rays traverse dry air without perceptibly heating it; obscure rays reflected by the earth raise the temperature of the strata that envelop them. This air, made lighter by surface evaporation and its own dilation, has a tendency to rise to the upper regions. When the phenomena remain the same upon vast spaces, the surfaces of equal expansive force remain parallel with the earth ; but the least disturbance, the least difference in the temperature of two neighboring spots may upset the equilibrium and determine a center of activity at the point where the upward pressure is greatest. The fact that heated air ascends is questioned by none; in order to demonstrate it, it is only necessary to place the hand above a lighted candle and to compare the sensation felt with the heat received when placed at the same distance from the light but in a lateral position. This difference results principally from the shock to the skin, of air molecules and carbonic acid endued with great upward velocity. And yet the products of combustion, in even temperatures, are heavier than air.
By the mere fact of its ascension the air becomes cooled; it is then that the vapor of water interferes to give by condensation renewed energy to the flagging movement. This vapor, first condensed in the form of aqueous vesicles or bladders, constitutes the clouds with their ever-varying forms; higher up, those clouds are composed of fine icy needles; it is the region of the cirrus; finally there exists, well within the confines of the atmosphere but at heights yet undetermined, a limit zone of the vapor of water, at the point where the temperature is such that the elastic energy of this vapor is null, and a limit zone of the clouds at the point where the density of the air is insufficient to maintain in suspension these meteors. If the clouds ascend still higher, this is due to the momentum of their ascending motion; and when this momentum is exhausted, they immediately descend towards the earth until they find a proper surrounding density.
Doubtless, at these limit zones stops the fluctuation of the vertical motions of the atmosphere.
INFLUENCE OF THE VAPOR OF WATER UPON THE CHANGES OF TEMPERATURE.
The water disseminated throughout the atmosphere under the forms of clouds or vapor acts like a heat moderator; it lessens considerably the variations of temperature, which would be excessive without its intervening agency. During the day the vapor of water absorbs in its formation a notable portion of the solar heat; at night the clouds hinder the dark radiations from the earth towards the planetary spaces and thus prevent the cooling off; when the sky is clear, the vapor of water yields to the surrounding air, by condensing into dew the heat that it held. But this precious characteristic is not without its drawbacks at the point of view of variations in pressure.
This condensed vapor, in fact, maintains in space a pressure which becomes lessened, and the level of the mercury is lowered by a quantity equal to the decrease in the tension of the vapor of water. Equilibrium can be re-established only by surface atmospheric displacements whose energy is in direct proportion to the depression.
In reality the barometric fall is inappreciable as long as the changes in the physical state of the water take place slowly, by a sort of relaxing process, so to speak, as in the diurnal phenomena we have already pointed out. It is different, however, when these changes are caused by a sudden cooling off similar to that produced by the inflow of the cirrus in our immediate atmosphere. In the latter case the fall is very marked, and the consequences are a surface atmospheric disturbance more or less energetic according to the copiousness of the condensations.
The above expose may explain the segmentation of cyclonic disturbances into the secondary whirlwinds that form in the proximity of the main depression and move parallel with its track. We shall recur to this subject farther on.
INFLUENCE OF CONTINENTS UPON THE MOVEMENTS OF THE AIR.
When one seeks to infer a general law from pure theory combined with observations of facts, he should as much as possible lay aside all the causes of exceptions. Those causes are numerous in the study that engages our attention; they are the continents.
The ideal globe in which the wind system would possess perfect regularity should be a sphere destitute of land, and whose equatorial plane would constantly blend with the ecliptic plane; the whole would be symmetrical with the equator and any meridian whatever. Those conditions are far from existing, and the problem is much more complex.
Upon continents, causes variable in the extreme, arising from the nature of the soil, its form, its elevation, its emissive power, the vegetation with which it is covered, etc., have a constant tendency to modify the state of the temperature, and as a consequence the system of pressures.
The resistance that continents present to the motions of the air must necessarily exert an influence upon the velocity as well as the direction of the winds; and if, in order to deduce a general law, we were to consider only the limited spaces occupied by these lands, the results obtained would be justly considered doubtful.
When sailing over the oceans, especially the largest ones, one is struck with the harmony that reigns in that apparent chaos, and feels naturally inclined to seek the primary causes.
This is the task we shall undertake, now that we know how to assign the atmospheric disturbances to the differences of pressures, and have specialized the forces that act upon the level of the barometer.
RESUME OF THE ATMOSPHERIC MOTIONS OBSERVED ON THE SURFACE OF THE GLOBE.
The wind system is nearly symmetrical in regard to the thermal equator, which oscillates a few degrees with the declination of the sun, north of the geographical equator.
In the regions between the tropics and the equator blow nearly constant winds, which have received the name of "trades," and which blow from the northeast in the northern hemisphere, and from the southeast in the southern hemisphere. The trades of the two hemispheres are separated by a calm belt; their polar limits oscillate with the declination of the sun.
The zone occupied by the trades is sometimes crossed, notably during the shifting period of the isothermal lines, by whirlwinds forming complete circuits, the winds shifting in an inverse direction of the hands of the watch in the northern hemisphere, and with the hands of the watch in the southern hemisphere.
These whirlwinds or cyclones are incited with a motion of parabolic translation whose concavity is turned to the east.
Higher up the trades, begins the belt of variable winds, in which the predominating winds blow from the southwest in the northern hemisphere; from the northwest in the southern hemisphere. (These winds have received the name of "counter-trades.") In this belt, the winds move generally (at least, in appearance to an observer standing still) in a contrary direction to the cyclonic rotation above referred to.
Finally, higher up, a limit not easily defined, but supposed to be near the 40th parallel, the winds blow nearly constant in a direction comprised between north and south, through west. This zone is called the belt of the general western winds.
We shall now try and explain all these movements and deduce a few conclusions relative to navigation in the belt of the general winds.
INFLUX WINDS (VENTS D' ASPIRATION).
When a communication is opened between two rooms of unequal temperature, an inferior current of cold air flows towards the thermal maximum, while an upper warm one flows in an inverse direction. The difference in temperature creates a difference of pressure. In order to re-establish the equilibrium, a displacement of the air is necessary—the cold air, being heavier, flowing below; the warm air, lighter, above. The whole solar action on the surface of the globe may be summed up in these few words.
The most remarkable cases of winds of "aspiration" or influx air currents are the land-and-sea-breezes, the trades, and the monsoons.
At a great many points upon the shores of the oceans there have been noticed two inverse atmospheric motions in the course of the same twenty-four hours. During the day the lands get more heated than the seas; the layers of air that envelop them become dilated, and, rising to the upper regions, create an inflow whose center is at the same time the thermal maximum and the barometric minimum; the sea breeze that rushes in to fill the vacuum begins to blow in the early hours of the afternoon. During the night, on the other hand, the land loses through radiation more heat than the sea, the barometric minimum shifts conjointly with the thermal maximum, and the flow of air seawards begins in the early hours ol the morning. In either case the return of the heated air takes place through the upper regions of the atmosphere.
TRADE-WINDS AND MONSOONS.
The trade-winds come from the same causes: the annual mean temperature is highest under the thermal equator; the air, extremely dilated and saturated with vapor of water, produces a barometric minimum—a relative minimum, of course; it is therefore another center of incitement for the strata of air situated to the north and south.
The monsoons are the exclusive results of the disturbing influences due to continents; they consist in land-and-sea-breezes lasting several months.
OBLIQUITY TO THE MERIDIAN OF THE TRADE-WINDS AND MONSOONS.
The trades being occasioned by the difference of pressures, which themselves result from the inequality of temperatures, should always move parallel with the meridian, since this inequality is most marked on the meridian. But all the parts of the globe revolve to the east with a velocity proportional to the cosines of their latitudes ; hence it results that the molecule which possesses at its point of departure a given velocity is behindhand on the successive parallels that it crosses to reach the equator; its relative motion is, therefore, oblique to the meridian. This is why the north trade blows from a quarter Hearing northeast, whilst the south trade blows from near southeast.
MONSOON OF THE INDIAN OCEAN; RETROGRADE MOTION.
During the northern summer the rarefaction of air produced by the overheating of the vast plains of Central Asia transforms the northeast trade of the Gulf of Bengal into a southwest monsoon. It must be noted that the extending motion of the monsoons is a retrograde one; that is, it takes place inversely with the direction of the wind. Thus, when the southwest monsoon of the Indian Ocean sets in, it is first met with in the northern parts ; it progresses backwards; it is observed at Calcutta earlier than in Ceylon, and in Ceylon earlier than on the equator; every day it is 15 or 16 miles farther to the south.
This motion, so characteristic, defines quite accurately the breezes of "aspiration," and justifies the name. Let us also observe that the monsoon of the Indian Ocean partially destroys the symmetry of the wind system relatively to the equator. In the Mozambique Channel the winds in October predominate from south-southwest, south, and southwest, and continue to blow from those quarters down to the equator. They thus join off the coasts of Africa the southwest monsoon, which reigns north of the equator in the sea of Oman and Gulf of Bengal. This is not an exceptional case, but is the largest body of air that establishes a communication between the two hemispheres.
IMPETUS DUE TO THE ROTATION OF THE EARTH; IMPELLED WINDS
When we sail up the tropics the unequal distribution of heat over the surface of the earth is insufficient to explain the atmospheric disturbances. The temperature of a spot, in fact, is solely a function of the latitude, setting aside the continents; and if it be true that the force of the flow is less as we get farther from the equator, it is no less true that the "aspiration" works from place to place gradually, so that the air column of a given parallel is drawn towards the warm regions through the vacuum created by the moving columns of air of the intermediate parallels.
Why then do we see this motion stop on either side of the equator near the 25th parallel? It is because the mean barometric pressure keeps constantly increasing from the 25th parallel to the equator, and constantly decreasing from the 25th to the polar regions; yet the air of the higher latitudes is colder, less saturated with moisture, and weighs more to the same volume than that of the lower regions. It is then necessary that the thickness of the atmospheric stratum go on decreasing from the limits of the trades to the poles—in other words, that the pole be rarefied. The cause of this decrease is entirely in the centrifugal force occasioned by the rotation of the earth upon its axis.
Let M be a point whose latitude is l. If F be the centrifugal force developed at the equator, that at the point M will be F cos l, whose tangential component is Fcos lsinl or ½ Fsin2l. This component is not an unimportant quantity, since the centrifugal force at the equator decreases the gravity by 1/289; nothing at the pole and at the equator, it attains its maximum value under the 25th parallel.
In reality this component has no action on the bodies placed on the surface of the earth; owing to the elliptical form of our globe, the resultant of the centrifugal forces and gravitation is normal on the surface of still water. Are the conditions the same in the atmosphere? In other words, is the resultant of all the forces that pass through a given molecule normal to the envelope that contains it—an envelope that can be conceived as perfectly moulded on the form of the globe? This indeed would be the case, and the gaseous envelope would have no relative motion to the earth, the angular velocity remaining the same to the extremities of the aerial radius, if the sun did not determine in the mass vertical indraughts (aspirations) along this radius, and as a consequence, displacements in the direction of the meridian.
The effect of the sun being to change constantly the level of surfaces of equal expansive forces, the atmosphere cannot remain indifferent to the tangential component of rotation; but at what height does this force find the most favorable field to exercise its action?
It appears rational that close to the earth, friction, the pressure of the upper strata, and the cohesiveness of the molecules to each other, raise an impassable barrier against the tangential component; but the situation is quite different in the upper regions of the atmosphere, where the air, being lighter and freer, is endued with extreme mobility, and yields unrestrained to every impulsion. Moreover, its lineal velocity of rotation, and consequently its centrifugal force, is greater than on the surface. The air of these regions is therefore drawn towards lower latitudes, and this impulsion is marked, as shown by the barometric curve, by an increase in the density of the atmospheric envelope from the pole to the 25th parallel. The difference in pressure between these two points represents the work accomplished by the tangential component of rotation.
It is evident that air cannot keep thus accumulating towards the lower latitudes without seeking an issue. On the other hand, the rarefied regions of the pole become in turn indrawing centers, and it is for this reason that we see predominating, from the limits of the trades up to the higher latitudes, surface breezes that blow from southwest in the northern hemisphere, from northwest in the southern hemisphere. These breezes start at the north and at the south, according to the hemisphere; the rotation of the globe then modifies their initial direction, because they are in advance, upon the successive parallels that they cross in reaching the poles. To this primary cause of the prevalence of the western winds in the extratropical belt must be added the tendency to the algebraic equilibrium of the impulsion of the air.
Here a few words of explanation are necessary. If the earth were isolated from the sun, in order that the period of gyration around the axis should be constant, it would be necessary that every aerial displacement, in whatever direction, caused by an interior force, should have for a corollary a motion equal in quantity and in an opposite direction. Now, it is admitted that the earth throws off from its bosom towards the interplanetary spaces a quantity of heat at least equal to that it receives from the sun. The forces developed by this luminary may, therefore, be considered as interior forces in the interval when the quantities given out and received are equal: we have then the right to apply to the whole of the forces that govern the atmosphere the theorem of the interior forces, and to establish as a principle that the algebraic sum of the impulsion given to the aerial envelope tends to zero, and becomes strictly null from one year to another.
The trades of the two hemispheres cover a belt representing nearly two-fifths of the whole surface of the globe. They have a very marked west component, due, as already explained, to the rectilinear velocity eastward, which attains its maximum at the equator. Having reached the center of indraught, these great gaseous masses annul their polar component and, rising in the atmosphere, continue to move westward by virtue of their acquired velocity. The movement is all the more marked, as the height they reach is greater. It is only by slow degrees that they swerve towards the poles to become decidedly southwestward and northwestward, according to the hemisphere. It will be readily seen that in this immense area up or down, the motions of the air in the majority of cases are westward.
In order that the equilibrium of the impulsions be a fact, it is necessary that in the extratropical belt the motions of the air be in a great measure directed eastward.
[Let us note incidentally, that the seasons when the western winds are stronger and most prevalent correspond with the winter period, when the trades are more intense and less intermittent.]
Such are the origins of the counter-trades and western winds in general. It seems difficult to explain otherwise than we have done the distinguishing character of the counter-trades, which is to move in an inverse direction to the breezes of " aspiration " from the warm latitudes to the cold ones. It has been accepted up to the present that these winds are caused by the landing of the return upper trades. Such being the case, how would the surface trades be fed, and how explain the barometric maximum occurring at the very point from which the trades and counter-trades start on their opposite journey? The theory of the return of the two upper currents, on the contrary, furnishes us with a rational explanation of that maximum; thanks to this very meeting, the trade and counter-trade are naturally fed. And now let us study the origin and formation of the normal whirlwind or anticyclone.
But first, we must mention a secondary effect of the tangential component of rotation, an effect that has recently been pointed out by the eminent American meteorologist, W. Ferrel, and which may be expressed by the following formula: All aerial currents flowing eastward tend to the equator, and all currents flowing westward tend to the nearest pole.
When the atmosphere is at rest relative to the earth, the centrifugal force with which it is endued is the same as that of its contact parallel; but if its velocity eastward or westward be greater or less, its centrifugal force, and, as a consequence, the horizontal component of this force, will be superior or inferior to that of the same parallel. The aerial current will therefore assume in regard to the earth a relative motion whose direction will be governed by the difference in the centrifugal forces, towards the equator, if moving eastward, towards the pole, if moving westward. With these conditions, the law of the wind-gyrations appears to be in plain conformity with the phenomena that come under our daily notice. In the northern hemisphere the winds revolve like the hands of a watch; in the southern hemisphere in an inverse direction.
NORMAL WHIRLWIND OR ANTICYCLONE.
The barometric curve points to the existence of a maximum upon the parallels near the tropic; on the other hand, we know that this line is the starting point of the trade going towards the equator and the counter-trade going up towards the pole. The apparent anomaly of this maximum and flow of air in two opposite directions can find an explanation only in the meeting of the superior return currents: the return of the trade in the upper regions of atmosphere, and the impulsion of the polar air towards the equator, also in the upper regions.
How will the meeting of these two upper currents take place, and what gyratory movements will they originate? Both are evidently oblique to the meridian, and both possess a tendency to gyrating due to the tangential component of rotation. Considering only the southern hemisphere, we see that the direction of the polar current is from the southeast, with a tendency to blow from a point of the compass that removes it farther from the equator, east-north-east; the direction of the equatorial current is from the northwest, with a tendency to blow from a direction nearer to the pole, west-southwest. If we suppose that the impulsions are destroyed by the meeting of these two adverse currents, there remains present only their tendency to gyrate in the same direction, and the normal whirlwind is formed. This influx of air from two opposite directions will seek an issue in a vertical direction above as well as below; hence a descending current that feeds the trade and counter-trade, and an atmospheric inflation clearly demonstrated by the barometric maximum noted on the curve.
Such are the conditions under which the two upper currents will meet: around the center the breeze revolves in an inverse direction to the hands of a watch, the polar current to the right, the equatorial current to the left.
Fig. 3 represents the sum-total of the determining forces of the movement in the upper regions of the atmosphere, but the breezes that come down to us on the surface from either side partake at the same time of the enveloping motion of the current to which they belong and the indraught energy in the direction of the radius, occasioned by the difference of pressure existing between the barometric maximum and the surrounding spaces. It is, in fact, known from experience that the winds that contribute to an area of high pressure have a marked centrifugal tendency.
The wind charts of Commander Brault, published by the "Depot" and bearing Nos. 33S3 and 3384, point out the existence of similar gyratory motions—the one around the Azores during the northern summer, the other around Tristan d'Acunha during the southern one. Around the Azores the breeze revolves like the hands of a watch; around Tristan, in a contrary direction. These two symmetrical gyrations agree perfectly with the barometric maxima described at those two points during the above-mentioned seasons.
SUMMER INFLUENCE.
These movements are principally noticeable during the summer. In this season, in fact, the difference of pressure between the higher and lower latitudes becomes less, the atmosphere expands under the beneficent influence of the solar rays, and the tangential component, whose action as we have said affects the upper regions only, carries towards the equator a light atmosphere, whose energy is feebler than in winter; everything takes place, therefore, as if the density of the atmospheric envelope had increased in the high latitudes. Moreover, the flow of the trades is diminished in consequence of a greater uniformity in the distribution of heat; the upper return trade is therefore less energetic, all the movements are more quiet, and the two upper currents, instead of rushing to fill a vacuum and create formidable whirlwinds, oppose one another from the effect of their acquired velocity, and form a calm area and a barometric maximum upon their meeting parallel.
STABILITY OF THE NORMAL WHIRLWIND.
The volume of those great masses of whirling air has no translatory motion, and there is no reason why it should. If the supply of the equatorial current is more abundant than that of the polar current, it seems that the latter, traveling a greater distance and distributed over a larger area, must be endowed with a greater velocity, and the energy of the two is thus equalized.
ROUTE OF A SHIP IN A NORMAL WHIRLWIND.
A ship running east-southeast in the South Indian Ocean, and meeting the cyclone on the west side, will notice the breeze working from north to south through the west, the barometer remaining high, but lowering somewhat with the northern winds, to rise again with the southwest winds. If the route of the ship should cause it to sail too close to the center, it will meet with calms characterized by a barometric maximum.
All these results perfectly agree with the data furnished by experience.
Conclusions.—These normal whirling motions are not fleeting meteors like the cyclones; their diameter is considerable, and the conditions they determine in a given region may endure for months. These normal whirlwinds or anticyclones cause great changes in the temperature—extreme heat in summer, intense cold in winter; this is caused, apparently, by the dryness of the air columns descending from the upper regions.
ABNORMAL WHIRLWINDS OR CYCLONES.
The cyclone is a whirling motion of the air in which the breezes revolve in the same direction in the same hemisphere. In the center of the vortex, characterized by a great barometric depression, there exists a relative calm; this central portion acts like a flue for the air streams in motion. All these whirlwinds describe a parabolic trajectory whose concavity is turned eastward.
Where do they originate, and to what agency do they owe their formation? Cyclonic movements are met with in all latitudes; they reach the coasts of France and England only after crossing the Atlantic from southwest to northeast, or from west-southwest to east-northeast; but the breezes revolving around their center are far less energetic than in the intertropical regions. Cyclones appear to originate in the neighborhood of the thermal equator, particularly at the time of the change of the monsoons in those belts where there exists a double annual system, and at the time of the displacement of the isothermal lines in the zones where the trades blow free. For instance, in the South Indian Ocean these storms occur most frequently during the months of February and March, at the epoch when, the sun ascending north, the trade winds, weakened and disturbed during the summer period, resume again the energy and regularity they will preserve during the whole winter. In the Gulf of Bengal, cyclones manifest themselves only at the time of the change of the monsoons, and those that make their appearance at the beginning of the northeast monsoon are more powerful and destructive than those that spring up with the southwest monsoon in May.
It seems from the above instances that cyclones are due to a disturbance of the dynamical equilibrium of the atmosphere, a disturbance caused by a change in the conditions of the distribution of solar heat at the surface.
Theories of the Cyclone.—In later years the theory of cyclones has excited a lively interest among the learned, and has given rise to two contrary opinions: the theory of "aspiration" or ascending motion; the theory of descending motion, due to M. Faye. Both theories have their fervent adherents; it is not our province to solve the controversy; we shall simply sketch the main lines of the dispute and point out that neither theory is exempt from scientific criticism.
Theory of Aspiration.—The older in point of date, and also the one that rallies the most adherents, is the theory of aspiration. We defined it in a few words at the beginning of this study.
When a center of depression is created at a given point, owing to a more active evaporation, to the superheating of the air masses that envelop the earth and find their expansion in the upper regions, and owing also to the vacuum that the incipient monsoon creates behind it at the moment when the dying monsoon still keeps on its course—the air is solicited on all sides with an energy proportional to its expansive capacity, i. e., in most cases to its density. As the density is maximum at the surface, the equilibrium of pressure tries to reform by renewals of surface air. But the directions of the inflowing breezes are modified by the rotation of the globe, so that these breezes, instead of directly reaching the center and filling the vacuum, can do it only after a succession of spiral movements. The centrifugal force developed by the curving of air currents contributes to perpetuate the movements by moderating the influx to the center. The gaseous masses that finally penetrate into this flue chimney must acquire, at least at a certain altitude, a considerable ascending energy, if we are to judge from the great heavy clouds that pour down upon the whole periphery of the vortex after visiting the upper regions. During this journey the liquid drops have turned into fine icy needles. When their energy in centrifugal flight, as well as in height, is well expanded, they incline earthward by the law of gravity, and cause in the spaces surrounding the trajectory of the cyclone atmospheric disturbances, such as thunderstorms, waterspouts, and secondary depressions.
Cyclonic Translation.—Cyclones change place; they move in a parabolic curve whose concavity is turned to the east. But here rests the theory of aspiration, powerless to define the reason why in the intertropical zone the whirl appears to be drawn by the upper air masses instead of following the motion of the lower stratum.
THEORY OF THE DESCENDING MOTION.
M. Faye compares atmospheric disturbances, such as waterspouts, hurricanes, cyclones, etc. (the magnitude of effects depending solely on the energy of the forces at work), to the whirling motions forming in streams, from the inequality of velocity in the surface, liquid threads or currents. "When in a stream there occur differences of velocity between currents flowing side by side, there is a tendency to form at the expense of those inequalities a regular gyratory motion around a vertical axis. The spirals described by the molecules are perceptibly circular and central with the axis; they are, to speak more correctly, like the threads of a screw slightly conical and inclined downwards, so that by following a molecule in its movements one sees it revolve with rapidity around the axis, which it nears little by little with a descending motion much less rapid than the lineal velocity of rotation."
According to the above theory, therefore, the origin of waterspouts, tornadoes, hurricanes, cyclones, etc., would have nothing in common with the physical phenomena we notice on the surface of the globe; it would exclusively proceed from a difference in the lineal velocity of air currents moving above our heads, and ipsofacto be a pure caprice of the elements. Everything would take place as if the confines of a mass of air in motion and the vacuum were as decided as the surface of a stream of water; as if the density of the air were uniform from the top of a whirlwind down to the bottom, the same as in the hydraulic mass.
We have seen that the origin of cyclones is intimately connected with the regime of surface temperatures.
It is also well known that storms are more frequent in spring, the season when the isothermic lines are suddenly disturbed, than at any other time; that hall falls far more frequently in the daytime than at night, and that the maximum of falls corresponds with the diurnal maximum of temperature, towards 2 o'clock in the afternoon.
Finally, the season of the pamperos in La Plata, and tornadoes on the African coasts corresponds to the period of excessive heats.
It seems to us, moreover, that a vast funnel, as described by M. Faye, can be formed only concurrently with the existence of a vacuum above our heads, and the vacuum, if it exists, is found at an altitude of 300 kilometers. In the contrary case, every attempt at a partial vacuum will be opposed by the upper strata flowing down vertically; it would therefore be necessary to admit that the whole column of air is sucked up by the whirlwind to that prodigious height where the atmosphere is, so to speak, imponderable. On the other hand, the descending column, as represented to us, is composed of air currents inferior in density to the strata through which they must pass. Here is something abnormal that the mind cannot easily conceive. Does it not require a steel point to pierce bronze or iron? And would it not be consistent to suppose that under the pressure of the gaseous masses flowing from top to bottom, the level of the mercury should rise? Now, it is just the contrary that happens.
Those are not the only objections that can be raised against M. Faye's learned theory. In order to explain away gyration, which is always the same in the same hemisphere, we are simply told: "the direction of the rotation of cyclones may be attributed to the fact that in these extremely curved currents, velocity diminishes transversely from the concave edge to the convex." Contradiction is very difficult owing to the impossibility of verifying the assertion, but the explanation is not one that will fully satisfy the mind; it is a statement ipsofacto.
Finally, one last objection: in all the cyclonic disturbances that visit our shores (Europe), the winds felt at the surface have a decided centripetal tendency. It suffices, in order to make sure of the fact, to cast a glance upon a synoptical chart of isobares when a depression traverses Europe from west to east; the breezes converge towards the center as if a regular suction took place there. In the hypothesis of the descending currents it would seem, on the contrary, that the moving gaseous masses should expand, in diverging, from the base to the periphery.
It should be stated that lately M. Faye has made an important concession to the theory of "aspiration": he admits, a priori, "two cyclones with very different depressions," the ordinary intertropical cyclone and the fixed depressions, "where the succession of phenomena works quietly; it is a question of static meteorology. There are formed towards the periphery more or less convergent breezes (regularly deviated by the rotation of the globe), but no violent gyrations. The air ascends slowly in them."
In the following sitting, M. Mascart, the learned superintendent of the "Bureau Central M6teorologique," whose opinion in such matters is too weighty to be passed over in silence, took notice of that declaration in the following terms: "With the exception of the meaning attributed fixed depressions, and the relative importance of the phenomena, this is a new concession that I am happy in recording. I should be pleased to hope that our colleague will go a step further, and, yielding to the evidence of facts established all the world over, recognize that the partial convergency of the wind in depressions is the general rule, as well in cyclones of all kinds as in the mean annual or season effects."
We must add, before finishing this cursory glance at the theory of descending motions, that if the various criticisms we have formulated be swept aside, this theory explains quite satisfactorily the movement of translation of cyclones and their segmentation into secondary whirlwinds. This is one of the most important points where the defenders of the "aspiration" theory fail obviously; none of the explanations upon this point published up to the present, at least as far as we are aware, fully satisfies the mind.
M. Faye has, after all, stirred up a world of new ideas and fixed the minds of the learned upon a question worthy in the highest degree of their attention.
After all, what is of importance to the sailor is not the "why" but the "how" of cyclones. It is to his highest interest that he should be filled with the idea that cyclonic movements of closed circuit are met with in all parts, that he learn to recognize their approaches under the higher latitudes, and manoeuvre in view of his objective point instead of resigning himself to kick about on the same spot from sheer ignorance in regard to the phenomenon.
What is also essential to remember touching intertropical cyclones, by far the most dangerous, is that the almost perfect circularity of the wind around the center is admitted for these storms by many eminent meteorologists. M. Faye in this respect is very positive. M. Mohn, a partisan of the adverse school, expresses himself thus: "In the revolving storms of the tropics the greater pordon of the movements of the air is performed circularly around the center; but the great velocity causes an extraordinarily considerable mass of air to pass constantly from the exterior into the interior of the hurricane, whilst the latter surrounds the vortex. Thus, therefore, there must exist in intertropical cyclones, the same as in our own cyclones, an ascending air current accompanied with all the phenomena that are inherent to it and sustain it."
Such being the case, the rules we are acquainted with are all that can be desired, and it will always be possible, provided the structural strength of the ship will allow it, to sheer off from the center in order to avoid the greater force of the hurricane.
We must, however, make some reservations in regard to one of those rules set down by Commander Bridet in his work on hurricanes in the Indian Ocean. It is known that the approach of the cyclone off the island of Reunion is indicated by a violent freshening of the breeze, besides the fall of the barometer and the leaden gray clouds that sweep over the sky. Admitting as a principle that the center of the disturbance is always at right angles to the wind. Commander Bridet advises captains who notice the trades increasing in freshness without changing direction, to bear away so as to cross the path of the cyclone forward of the center, and to keep on this course until the barometer shows a decided rise, and the wind begins to veer to south and southwest. At this moment, bring to on the starboard tack.
The above manoeuvre would be perfect if the air currents described an exact circumference; but it has been pretty well ascertained that, at least within a certain distance, the winds converge partially towards the center of the inflow. The ship that would strictly follow the above rule would be in danger of running into the cyclone, and perhaps passing through the very center that she was trying to avoid. To provide against this grave danger it would be only necessary to bear up two or three points, i. e. to steer west-northwest or west instead of northwest; but every ship will not be able to steer that course, and the advice applies only to strongly built vessels; the others will simply come to on the port tacks and maintain their position if the wind veers to the east and north, and put about, on the contrary, if the wind veers to the south and west.
GENERAL WESTERN WINDS.
When we sail higher up than the 40th parallel, the atmospheric circulation on the surface hardly varies except from north to south through west, or the reverse according to the hemisphere.
In these parts (Europe), as has been admirably shown by M. Lephay, the west winds are an indication and the consequence of a depression crossing the Atlantic from southwest to northeast. Around this depression the winds revolve in an inverse direction to the hands of a watch, so that an observer standing still may see the breeze veer from southeast to northwest through west, if he is to the right of the path; from east to northeast, and northwest if he is to the left. It happens in most cases that on the left edge of the cyclone, which is the manageable half-circle, the winds remain feeble, even blow from the west, the velocity of translation being greater than the gyratory motion. The center of the disturbance generally passing south of Ireland going towards Central Russia, it follows that the French coasts are swept by the right edge of the cyclone, on which the breezes blow from southeast to northwest passing through west.
Must we adopt for the whole of the zones of the so-called general western winds the conclusions reached by M. Lephay in regard to the Atlantic, to wit, "that the existence and vagaries of a well established aerial current are proofs of the passage of a cyclone in the neighborhood"? We do not think so. Certain well observed western gales, for instance the strong winds on the Needle Banks during the southern summer, present but a slight resemblance to cyclonic disturbances. On the other hand, what mostly strikes the navigator in the Indian Ocean or the Pacific is the persistence or rather the permanency of the western winds. Some ships have sailed over the immense extent of seas that reach from the first meridian to Tasmania without encountering an eastern breeze even for an hour. There must then exist in our part of the globe conditions that can be explained only through the influence of continents. We are too near the Gulf of Mexico, where cyclones have their origin, and the American continent is too close to allow the winds fair play in the interval that separates us.
In our opinion, the forces we have considered above are the only ones that contribute to the great aerial circulation, and cyclones in general are but accidents that may modify the foregoing laws without impairing them in the least; perhaps even some depressions are formed on the spot by the alternate action of the western and eastern winds. Let us consider only the southern hemisphere. We have seen that the meeting of the upper return trade and the breezes brought down from the pole through the horizontal component of the centrifugal force determines an atmospheric inflation upon a pretty extensive belt; the difference of level between that belt and the cold regions determines a flow of surface air towards the pole.
These winds begin at north, then shift to northwest in consequence of their lineal velocity eastward, and inflect more and more to the west and southwest, as much owing to the tangential component as to the general law of algebraic equilibrium of the motion quantities. If these winds renew themselves constantly with the same gyratory motion, it is because the forces that rule them are constant, independent of the temperature, and because they derive from the rotation of the globe around its axis a movement that is uniform and constant: We even believe that the anti-trades (southwest in the northern and northwest in the southern hemisphere) would be as regular and as permanent as the trades, if their air channel did not go on narrowing, whilst that of the upper wind of impulsion, on the contrary, goes on widening; in other words, if the relation of surfaces to be attained was not zero or infinity. In the relative conditions in which they are placed, the performance of these two superposed winds can only be a succession of fitful inrushes seeking to realize an impossible dynamic equilibrium.
Let us suppose that we stand still on any spot whatever in a high latitude and observe what takes place on the surface. The winds having originated in the north to die out in the south, after passing through the west, have determined a rarefication in our west and a relative barometric maximum in the east. In virtue of the continuity of the pendular movements, the end sought has been overreached; therefore it becomes necessary to fill up anew this rarefaction that solicits the breeze on all sides. But while from the north or northwest fresh supplies are constantly arriving, on the southeast the air keeps on rarefying, owing to the flowing towards the pole of the whole lower mass; there is therefore no equality in the struggle about to begin.
Three cases may then present themselves: either the depression will be filled by the northwest winds alone, and the movement will continue as before, or else the southeast winds will be feeble and, after passing around east and north, will die out after a short while before the flow of the northwest winds ; or again, the depression may be such that the energy of the eastern breezes may not be unimportant, and the combined evolution of the two opposing winds around the center of depression will create a cyclonic movement that will sweep like the lower stratum from northwest to southeast. As may be presumed, the latter will occur only in the case of a marked depression, for instance, when the western winds have been very violent; thus one storm begets another of a different nature.
We repeat, that in our part of the world, where the solar action operates through the continents in quite a different manner, the surface meteorologic phenomena must present a different character; the anti-trade rises no doubt to a certain altitude, where it reigns supreme. It is, moreover, well known that in mid-Atlantic the west winds are more prevalent than near land, and that in winter they are uninterrupted. But in the south seas, where these disturbing causes are absent, where resistance to the movements of the air is almost null, atmospheric phenomena present a great regularity; this is confirmed by reports of captains and books of navigation, and we cannot do better than to compare the alternate action of west and east winds with the pendular motion of a ship well heeled over: when the reactions on the immersed side are very energetic, the ship, passing the vertical line, rolls to windward, but only to roll all the more heavily to leeward; the roll to leeward corresponds to the west winds, and the roll to windward corresponds to the east winds.
GALES OF WIND ENCOUNTERED BY THE TRANSPORT LA DORDOGNE.
It must not, however, be supposed, on the strength of what precedes, that sailing over the zones of the western general winds presents no difficulty and that the breezes are invariably favorable. The winds will sometimes sweep violently over them from the east. In such case one can easily judge whether he has to deal with a cyclone, and manoeuvre accordingly in order to save time and distance. This we propose to demonstrate from our own experience.
In a voyage around the world, from the latter part of 1883 to the middle of 1884, the sailing transport La Dordogne encountered within a month three successive gales in the southern hemisphere. Let us analyze them.
On the 11th of September La Dordogne was in 40' south latitude and 4° 34' west longitude ; the course was east-southeast, with a moderate breeze from northwest; the barometer, which had been rising for two days, showed 750.5 mm. During the afternoon watch the weather became squally, the horizon in the north thickened, the barometer falling 2 mm. About 6 o'clock, during a heavy squall, the wind shifted suddenly to east-northeast, blowing a gale. Night passed on, the barometer still falling, and the wind veering around little by little to the north, the ship was hove to on the port tack. The following day, the 12th, at 8 in the morning, the barometer stood at 734 mm., its maximum level, and the wind blew from north-northwest. During the entire day the barometer kept rising and the wind became more favorable; finally, on the 13th, at 8 in the morning; we continued our route east-southeast, with a fair breeze from the west. Later on, the wind backed to northwest, blowing a fine, steady breeze.
Fig. 4 represents the relative movements of the cyclone and the ship. Struck by the cyclone with winds from east- northeast, we sailed over a chord of the dangerous semi-circle. We assumed as the direction of the movement of translation of the whirlwind the position of the wind at the moment of the barometric minimum north-northwest to south-southeast.
It may be seen by the figure that the convergency of the wind is almost null as well at the isobares of the front part of the whirlwind as in the second part. We may conclude from what precedes that we crossed in its dangerous semi-circle a revolving storm going south-southeast.
Gale of December 25th.—On the 24th of December the Dordogne stood at noon in 43° 40' south latitude and 42° 32' east longitude, with a light breeze from east-northeast, the barometer at 754. At midnight the wind blew moderate from east-southeast, squally overhead, the barometer indicating 749 and falling rapidly during the night, wind freshening. At 6 in the morning we lay to on the port tack, wind very fresh from southeast, barometer 738. At noon the barometer stood 734, the wind blowing a gale from southeast.
From that moment the wind shifted to west and the barometer rose. At 8 in the evening the wind was from the west-southwest, blowing a strong breeze; barometer 745. Finally, at 10 in the morning of the 26th we hauled east-southeast with winds varying from southwest to west.
As in Fig. 4, we have chosen as direction of the path the position of the wind at the moment of the barometric minimum, but in Fig. 5 the air streams are decidedly convergent. We may again conclude that, sailing over a chord of the manageable semi-circle, we crossed a cyclonic disturbance moving to the southeast.
Gale of January 6, 1884.—For the third time we were warned of the approach of a cyclone through the same indications: rapid falling of the barometer, threatening aspect of the weather, specially in the north; winds from the east, fitful in strength and direction. The captain of the Dordogne, convinced that we were in the presence of a cyclonic disturbance similar to the two preceding ones, decided to pass around the storm on the west and seek north of it winds favoring our course to the east. Here is an abstract from the ship's log:
January 5th, position of the ship 44° 46' south and 70° 40' east, running east with a light breeze from west-northwest. During the day the wind shifts about; from west-northwest it veers to southwest, and suddenly jumps to south-southeast, then northeast, and, finally, towards 7 o'clock in the evening settles at southeast, freshening gradually; the barometer has fallen 4 mm. The course is set at north under shortened sail, with orders to keep the wind four points on the quarter, hauling to starboard as the wind veers aft.
This was done. During the entire night, the ship going under close-reefed foresail and fore-staysail, keeps the wind four points on her quarter; at 2 o'clock the barometer stands 475 mm. minimum point, the breeze blowing from south ; at 4 o'clock it blows from south-southeast; finally, at 11 o'clock in the morning we resumed our course east, one quarter south, with wind from the southwest; the sea was very choppy, an unmistakable proof of the passage of the cyclone. We lost 70 miles northing, but the winds were favorable, while on the western edge the fluctuation of the barometer was insignificant. Owing to this skillful manoeuvring, we saved, perhaps, 36 hours of lying to, and took advantage of favorable winds, which, in cyclonic disturbances in the southern hemisphere, are north of the adverse winds.
We will remark, in passing, that such manoeuvre is practicable and unattended with danger only when the ship is assailed in the manageable semi-circle, where the breeze is far less violent than on the other side. Besides, if from the beginning the ship is kept before the wind, there is very little need to fear that the convergency of the winds will drive the vessel into the calm center, owing to the east component of the movement of translation of the whirlwind, which takes the cyclone farther away while the ship heads a course near to north.
We cannot cite many examples of a similar manoeuvre, but there is as much value in one single experience as there is in a whole series, and there is no possible reason why a successful manoeuvre of the Dordogne would not prove equally so with any other ship.
What is most important to remember in regard to the preceding examples is that:
1. The whirlwinds of the high latitudes may form a complete circuit, i. e., have their share of east winds, which winds are adverse when sailing eastward.
2. The extra-tropical whirlwinds of the southern hemisphere follow a track lying between east and south, generally close to southeast. The manoeuvre of the able commander of the Dordogne is worthy of imitation, but how is one to know of the approach of the storm? By winds from the eastern quarters fitful in force and direction; by the abnormal gyration of the breeze; by a marked fall of the barometer; by the cirrus overspreading of the sky. We do not hesitate to say that the latter indication is perhaps the best.
Whether the cirrus are the causes or effects of cyclones; whether they are incited by the inflow of humid air into the central section, which on rising in the upper regions becomes cooled and then pours down over the periphery; or whether they are the source of an active agency and electricity on which these meteors feed—is a matter of little importance to us. What is far more interesting is to recognize them in time; for their intrusion in our immediate atmosphere, to which they tend by law of gravitation, will cause abundant precipitations, and it may happen that when we wish to examine the sky it is covered by a veil of low clouds.
For two years past we have studied attentively on the coasts of France the cirrus formation, and we can affirm that there is no better forecast, including the barometer, of storms, gales with rain, hurricanes, and bad weather in general. The abundance of coming rain and the violence of the wind may be reckoned by the apparent thickness of the layer of the cirrus; even in summer, when the fierceness of the sun's rays dispels the clouds, we have never observed a non-concurrence between the appearance of small, white, dense, and very high clouds and the subsequent coming of atmospheric disturbances.
RESUME AND CONCLUSIONS.
There exist in the belts of variable winds and general winds two inverse gyratory motions: the first, a normal whirlwind of vast extent caused by the meeting of two adverse upper currents. Its characteristics are slight barometrical fluctuations with a central maximum. The west winds are to the north of the east winds in the northern hemisphere; they are to the south of the east winds in the southern hemisphere.
The second, whose range of energy is much less, is the abnormal whirlwind. We think that it is a cyclone either formed on the spot, as we have explained, or in the intertropical regions, whence it follows the polar branch of its path. It is characterized by considerable barometric oscillations, with a central minimum. The west winds are to the south of the east winds in the northern hemisphere; they are to the north of the east winds in the southern hemisphere.
Independently of those two rotatory motions, the winds in the extratropical belts generally vary from north to south passing through the west, or inversely according to the hemisphere. These motions are due as much to the rotation of the globe as to the tendency towards the algebraical equilibrium of the impulsions of the air.
SELECTION OF A SAILING PARALLEL.
The first problem to solve for a captain on a long eastward voyage is the selection of the parallel he intends to follow. Without declaring ourselves in favor of this or that latitude, we shall simply remark that there exists a direct relation between the energy, the frequency of the anti-trade upon a given parallel, and the corresponding acceleration of the barometric curve; the greater this acceleration, the more numerous the chances of fresh breezes varying from the distant pole to the west. In crossing the Indian Ocean or the South Pacific we should then advise the selection of the belt in which the fall of the barometric pressures is most steep. With this idea in view, it will be found necessary to ascend at least as high as the 42d parallel, and avoid the zone comprised between 46° 30' and 50°.
CONCLUSIONS.
Once the route parallel determined upon, the captain need pay no attention to the gyration of the winds so long as they are favorable; he must take as much advantage of them as possible in reaching his destination. But if fortune should turn and he meet with adverse winds, several contingencies will present themselves. If the east breezes are feeble, the thermometer remaining high (see the mean on the curve), there is no cause for fear; those breezes will not last, and will soon become western after passing through north. If the winds are persistent, irregular in violence and direction, and accompanied by a marked fall of barometer, and if at the same time the sky becomes overspread with very high, small clouds, make ready for a revolving storm with winds from the eastern quarters at the outset.
When the cyclone reaches you, either the winds at the outset will be very near northeast, in which case you are in the so-called dangerous semicircle, and you must bring to on the port tack to await patiently until the breeze, which is bound to veer to north and then to west, has become sufficiently favorable to allow you to steer your course, or else the winds at the outset will be nearly southeast. In the latter case you are in the manageable semicircle, and you may sail to the north of the disturbance, where you will find favorable winds. You will have to round the cyclone on the west by standing to the north (perhaps one or two points west of north, in case the winds should delay in veering south); keep the breeze broad on the quarter, hauling course to starboard as the wind veers aft, until you are steering your course with winds varying from southwest to west. Finally, if with the winds southeast at the outset you find yourself unable to sail north, bring to on the starboard tack in order to keep the ship's head to the sea, and ride as easily as possible.