The naval officer in command of a battleship in a squadron has no problem of greater importance in the evolutions of naval tactics or at target practice than the determination of the distances of other vessels in the fleet for the distance of the target.
The inventive genius of the navy has devoted itself to inventions for determining distances, ranges &c., and some of these instruments are admirable, but none are at all times perfectly reliable. Fiske's stadimeter is an excellent instrument, his range finder has also in practice under favorable conditions proven to be very good, but for target practice the gun itself is generally conceded to be the best range finder. But in practice and especially in a naval engagement these instruments and means will not always be available and therefore every contribution to the subject of range finding or estimating distances should be diligently considered and practiced for successfully executing manoeuvres and conducting target practice.
When executive officer of the U. S. S. Montgomery under the command of Captain George A. Converse (at present rear-admiral and chief of bureau of ordnance) the writer was directed to carry out exercises for estimating distances. The captain sent the navigator out in the steam launch with a stadimeter to go to varying distances of from 2000 to 4000 yards. Upon arrival at predetermined distances the captain required the executive and divisional officers to estimate the distance of the steam launch. Gun pointers and sharpshooters were likewise all required to secretly report their several estimates of the distances of the steam launch. When this was first tried the estimates by different officers and men varied as much as 500 yards, but after a little practice the variations differed by less than 50 yards and a large percentage of the estimates were very close. This exercise was another evidence of the fact that practice makes perfect, and the great improvement in our target practice demonstrates this most forcibly. The terrible disaster recently experienced on board the Missouri is greatly to be deplored but notwithstanding the appalling risks, the necessity for such target practice is too urgent to permit us to entertain the thought of restricting target practice.
The advantages of this practice of the Montgomery at Pensacola before the war bore good fruit, subsequently in the destruction of Fort Canuelo at the bombardment of Puerto Rico, May 12th, 1898, where the Montgomery threw 314 5-inch shell from six 5-inch guns in a period of exactly five minutes, at the end of which time there was nothing left of that fort.
While in command of the U. S. S. Monterey in the Asiatic fleet last year (1903) the writer adopted a simple method of estimating distances by which officers and men became very expert is estimating distances very accurately.
This method consists of getting two lines of sight, one with the right eye and the other with the left eye. The observer simply sights with his right eye along the right forearm extended to its full extent and pointing with the right forefinger at the distant object. He then closes the right eye and sights with the left eye, holding the right arm and head rigid as before. In this case the second or left-eye line sight will point to the right of the object first sighted with the right eye, a distance equal to one-tenth of the distance that the said object is from the pointing finger of the observer's right hand. These two lines of sight intersect at the point of the forefinger of the right hand, and with lines, joining the two eyes and lines joining the object with the point to which the left-eye line of sight shall have moved to the right, form two right-angled triangles which are opposite and similar.
The eyes are normally 2.75 inches apart and the right forearm fully extended will bring the point of the right forefinger 27.5 inches from the right eye; the proportion of 10 to I exists between the base and altitude of the smaller right-angled triangle, and the same proportion exists between the larger triangle in which the base is the estimated distance that the left-eye line of sight shall have moved to the right of the object and the altitude is the distance of the object from the intersection of these two right-angled triangles.
An example of the practical use of this method will be clearly understood. While at Chefoo the Monterey was required to take position 300 yards from the Monadnock. The Monadnock was 55.5 feet beam and when directly astern of the Monadnock observer pointed at the mast of the Monadnock and observed that the left-eye line of sight moved to the right a distance equal to a beam and a half of the Monadnock, a distance equal to 55.5 feet plus 27.75 or 83.25 feet, making the distance of the Monadnock 832.5 feet. By pointing to the port edge of the after turret of the Monadnock, a point just 34.5 feet from the edge of the starboard rail at its greatest midship section, the distance was found to be goo feet when the left-eye line of sight pointed to the right at a distance such that the Monadnock might just fill the space between that point and the position of her starboard rail, a distance of 90 feet. It was required that the Monterey should be 300 yards or 900 feet from the Monadnock and every officer and man on board could at any time determine the distance.
The distance which the left-eye line of sight moves to the right of the previously observed line of sight with the right eye, is only an estimate and if that estimate is erroneous the distance will have been estimated thereby with an error ten times that of the first estimated lateral displacement. If this lateral error amounts to 10 yards the estimated distance will be 100 yards in error, but such an error will not be realized by those who are expert in this method. A little practice, especially in observing distances of objects of known dimensions will make the error in the estimated lateral displacement of the left-eye line of sight very small, much less than a foot, and therefore invariably give the distance Within 5 or 6 feet of the true distance, even when the object, such as a steamer at sea may be 2 or 3 miles distant.
A necessary corollary of this is that the length of a ship or any other object at a known, or previously determined distance, will also be given by observing the estimated lateral displacement of the left-eye line of sight and giving the estimated length of that object (for example, if a ship when broadside on) as one-tenth of her distance from observer.
This method of observing and estimating distances was used on board the Monterey while under the writer's command on the Asiatic station every day at setting up drill at morning and evening quarters. Divisional officers then required the men to estimate the distances of vessels in the harbor taking advantage of knowledge of the length and beam of the ships or of boats near by as might offer.
This method is not altogether limited to determine distances during daylight but with range lights at say, 300 feet apart when broadside on, the distance of the range lights from the observer might be very accurately determined at, say 15oo yards, when the lateral displacement of the left-eye line of sight should happen to be exactly one-half the distance between the lights.
While estimating distances on board the Monterey by the method used by Captain Converse it was found that the estimates of distances differed widely under different conditions of the atmosphere, whether hazy or in bright sunlight, but this method of the right and left-eye lines of sight will be found by practice to be quite independent of atmospheric conditions. The method is manifestly always available, a stadimeter may be left below, or out of adjustment, or otherwise not available, but every officer and man will always appear with his eyes and arms and intelligent use will by practice make him more or less independent of instruments. The gun is claimed to be the best range finder, but in an engagement it may and probably will be often impossible to determine where the shell from your gun struck, if other ships and guns are firing at the same time, and the writer knows of no instrument that can be depended upon to meet all the requirements that will arise in a naval engagement. This method is therefore earnestly commended as a most valuable aid for successfully carrying out manoeuvres in naval tactics and in target practice.
Since officers and men are not all uniform in size it is evident that the distance of the end of the right forefinger with the right arm fully extended in front of the right eye from the right eye, is not always exactly 27.5 inches, neither is the distance between the two eyes always exactly 2.75 inches; but the ratio between those two distances of ten to one always is or can be made to be the same. This ratio will have to be determined for each and every individual, and in case practical experiment should show this ratio to be wrong, the individual can readily make this ratio, either by prolonging the point of intersection of the two triangles by means of a lead pencil in case the ratio of the length of forearm and distance between the eyes is less than ten to one, or by using the thumb as pointer in case the ratio is greater than ten to one. The proportion of ten to one is the most convenient factor to use and each individual should be taught to ascertain his own personal range-finding ratio for estimating distances by two lines of sight from the right and left eyes.
In many scientific discussions on range finding it is the fashion to regard all mere estimates of distances as utterly unreliable, but estimates and good judgment are required even when using the accurate instruments of precision, and observers are required to depend upon their estimates of coinciding reflected images, etc. In the most scientific range finder which the writer has ever used, viz.: Carl Zeiss stereoscopic range finder, in which the stereoscopic plastic or depth of perspective is the principle, the observer is called on to estimate when the image of the distant object appears to be at the same perspective depth as certain figures on a scale blending into the perspective view of that distant object. Here good judgment is a requisite and observer must have two eyes of equally good visual power. It is therefore absurd to ridicule methods of estimating distances.
Careless observations of distances by this simple method of the two lines of sight with right and left eyes will never give reliable results. This method though simple requires rigid compliance with its conditions. The right forearm must be fully extended and directly in front of the right eye. When the right eye is closed and the left eye line of sight used the observer must be very careful to keep his arm and his head exactly in the same position, the very slightest deviation will cause a great error and vitiate the predetermined ratio of the distance between the eyes and the distance of the end of the index finger from the right eye. Again the observer must be extremely careful that the two triangles formed by the intersection of the right-eye and left-eye lines of sight are strictly right-angled triangles, and also that the point of the lateral displacement of the left-eye line of sight to the right of the object is a point in a line parallel with the line between the eyes or at right angles to the right-eye line of sight.
Again, in estimating distances by using known values of the length of a ship for instance, the observer must be sure that the observed ship is broadside on, or if not broadside on he must make allowance for the way she is heading. It is evident that if the observer is off the starboard quarter of a distant vessel 300 feet long for example, and if his first right-eye line of sight pointed at her stern and his second or left-eye line of sight pointed to her bow, the lateral left-eye line of sight would not have been displaced a distance of 300 feet, but only about 225 feet, and the distance in that case would be about 2250 feet instead of 3000 feet.
The greatest and in fact the only objection to this method of estimating distances is its simplicity. It is so very simple that few will take the trouble to rigidly adhere to its requirements, but to those who do strictly comply, practice will prove it to be much better than some instruments of precision that claim to be accurate range finders. This method may be said to be never liable to get out of adjustment, but that is not true, for every careless observation, when observer does not have his arm properly extended directly in front of the right eye or should move his arm or his head between the observations of both eyes, may be designated as being out of adjustment.
It is earnestly hoped that the naval service will take up this method and require every officer and man to practice it at every opportunity. It is always available, requires no apparatus and costs nothing but strict practice. But correct estimates will only be obtained by strictly adhering to the requirements. It is so easy that it is difficult to comply with its strict requirements.
The writer had the tops of the turrets marked with white lines as a dumb compass that facilitated taking bearings of distant objects from the bridges, that proved to be a very decided aid in estimating distances.
To insure accuracy in estimating distances it is advisable for divisional officers to have a right angle triangle constructed of light battens of dimensions such as to simulate the observer's small right angled triangle—
The base may have a small groove to fit over the nose. Then closing left eye observe along the edge with right eye, sighting on distant objects. Then close the right eye and sight along the hypothenuse with left eye and estimate how far the left-eye line of sight points to the right of the object first sighted.