When one considers the accuracy with which modern guns are constructed, the careful inspection and proof of the ammunition, and the ease with which the weapon is manipulated, he is naturally led to expect the best results in target practice. An examination of the target sheets is somewhat disappointing. We know that the gun used can put all of its shots within a three or four-foot circle at a thousand yards, that the men are carefully drilled at aiming and pointing, and we are then compelled to examine the third element in the problem of hitting the target, that is, the means we have provided for actually sighting the gun.
With the ordinary bar sights now in use the gun captain is called upon to see three objects in line, viz., the rear sight, 15 inches distant; the front sight, 50 inches distant; and the target, anywhere from a few hundred to several thousand yards away. To see three, or even two, objects distinctly at such widely varying ranges is an impossibility. The eye without conscious effort will accommodate itself to any distance, but it can only focus for one distance at a time. This can easily be shown by looking through a wire screen held a foot or two from the eye at some object several hundred yards away. The distant object will be distinctly seen, and the wires of the screen will be scarcely visible; and conversely, if we see the screen the distant object will only give a hazy impression. In sighting a gun, then, there must be a compromise effort to see all three objects at once, with the result that they are all indistinct, making the pointing largely a matter of practice and judgment, whereas the angle between the line of sight and the axis of the bore should be exactly what the sight bar calls for and totally independent of the personality of the gun captain.
Lack of acuteness of vision also militates against good practice with the ordinary bar sight. The front sight being a cone or point, and the rear sight a notch, the front sight would probably appear shorter and the notch less deep to poor eyesight than to good, again bringing in the personal element to the detriment of the gun practice.
It was not until 1801 that sights of any kind were used in aiming guns. Up to that time the men were instructed to look along the "line of metal," or exterior of the gun, and aim at some point of the enemy's ship above the point to be hit. When first introduced, both front and rear sights were fixed and merely gave a line parallel to the bore. This innovation took place nearly 500 years after the introduction of cannon, and was objected to then by as good a man as Lord Nelson. It was not until much later when a French army officer succeeded in introducing movable rear sights graduated to cable-lengths.
Until the beginning of this century it would perhaps have been superfluous to have introduced accurate sighting methods when the gun element had contributed so little to accuracy. The charge was measured out, and if the powder was a little old, a quart or so more was used. Guns were single shotted or double shotted with little thought of the effect on the trajectory. However, the optical defects of bar sights must have been apparent at an early date, for as long ago as 1857, Capt. Younghusband, of the British Army, introduced the telescope as a means of sighting guns. The telescope was mounted on a bar secured to one of the trunnions of the gun and parallel with its axis.
The telescope sight came into more extended use in 1875, when Major Scott, of the British Army, introduced his sight.
The telescope is attached to the gun until it is laid, and is removed just before the gun is fired. Its distinguishing feature is an arrangement by which it can be attached to or removed from the gun quickly. It is still issued to the English Army with their 12-pdr. field guns.
About eight years ago Captain Grenfell, of the British Navy, produced an arrangement by which the telescope was mounted on a curved sight bar, the curve being concave toward the trunnions of the gun. As the sight bar is raised the telescope is depressed through a corresponding angle equal to the required range, after which the gun is elevated until the optical axis is coincident with the target. This telescope is trained and elevated with the gun.
In 1890, Lieut. Fiske, U.S. Navy, obtained a patent on a telescope sight which had the distinguishing feature of being secured to the shield of the gun, and which depended, more or less, on the roll of the ship to bring the line of sight in coincidence with the target. This arrangement avoided all shocks to the telescope and permitted the gun captain to keep his eye at the sight while actually firing the gun.
The introduction of gun mounts in which the gun recoils in a slide or sleeve in the line of fire solved the difficulty hitherto present in all attempts at sighting guns on shipboard by means of telescopes. This permits us to mount the telescope to all intents and purposes on the gun. The eye can remain at the sight while the gun is recoiling, and all shocks to the telescope are avoided; the gun can be elevated and depressed without disturbing the setting of the telescope, yet the latter partakes of all the movements of the gun except recoil.
The Bureau of Ordnance design includes an ordinary terrestrial telescope fitted with cross-wires, having its optical axis parallel with that of the gun and arranged to oscillate in a vertical plane, or rather a plane making a small angle (equal to the drift angle) with a vertical plane through the axis of the gun, by means of a screw pressing against a rigid arm depending from the barrel of the telescope. Attached to this screw is a drum, upon the periphery of which are marked the ranges. As the drum is revolved to the higher range the screw moves back and allows the front of the telescope to depress. The optical axis is then made the line of sight by elevating the gun (and with it of course the telescope) until the target appears in the cross-wires, when the gun is fired. In the case of turret guns the sight is mounted on an oscillating table, which is made to move parallel with the gun by means of a straight rod, the lower end of which is attached to the slide on which the gun recoils, the upper end being attached to the table. This device produces a simple parallel motion between the telescope and gun, and is in effect the same as if the sight were mounted on the gun slide, as in the case of the smaller guns. It is inadmissible to mount it directly on the slide when the gun is in a turret, for the reason that at low ranges with the ship heeled, or when firing at elevated objects, too large an opening would be required in the turret for sighting through. Fig. 1 explains this.
Fig. 2 shows the method of applying the sight to a turret gun. ON is the arm on the oscillating slide. B is a rod connecting the arm ON to an arm of equal length keyed to a cross-shaft, to which the telescope table E is also keyed. Upon this table rests the telescope stand, so arranged that it can be moved in azimuth by means of the tangent screw t. It can also be adjusted in angular altitude by means of the screw a. After getting the telescope adjusted these screws are not disturbed until something has occurred to destroy the parallelism of gun and telescope. The rod B is provided with a turnbuckle, so that its length may be adjusted to equal the distance between the center of the trunnion of the gun-slide and the center of the cross-shaft.
Fig. 3 shows the telescope as designed for mounting on other than turret guns, the only difference being in the base of the stand. In this case the stand is stepped into a circular socket and clamped by means of a cam. By this means it may be readily shipped and unshipped. A is the securing screw for the diaphragm carrying the cross-wires. BB are the front and rear-finder sights. C is the stand. D, arm cast in one with the trunnion band, and is provided with a hardened steel piece E which takes against the point of the screw. Good contact is secured by means of the tension spring F. G is a drum made of aluminum upon which the ranges are marked. It is turned by means of the milled head H until the proper range comes next to the pointer J. The marks are arranged spirally about the drum, and the pointer is so shaped as to hide all adjacent marks. The pointer is secured to the stand by the screws KK; the slots at these points permit of a small movement of the pointer, so that in adjusting the sight in altitude it is only necessary to bring its optical axis parallel to the gun, after which the pointer can be shifted to the zero mark and clamped. The two bolts MM allow of a similar azimuth adjustment. The set-screw L is provided for the purpose of checking any tendency of the drum to rotate when the gun is fired. By setting up on it slightly the friction of the screw is increased at will.
The telescope is provided with an erecting eyepiece. Although this increases its length very much and doubles the number of lenses with a consequent loss of light, it is highly necessary that the gun pointer shall not be confused by working with an inverted image of the target.
The vertical cross-wires are double, one on each side of the axis, with a horizontal cross-wire exactly in the axis, all of them being firmly secured in place, so that no adjustment need be made for collimation. The wires are made much thicker than is usual, so that when the sight is used at night they will be plainly visible.
All lenses are firmly secured in place, and no focusing for distance is necessary. The object-lens is made about two inches in diameter so that as much light may be taken in as possible, thereby increasing the value of the telescope as a night sight.
The magnification is about 1.5, and the field nearly 17 degrees. Great care has been exercised to get as large a field as possible, and 17 degrees is the limit with the dimensions allowable. The field of the human eye is about 60 degrees, but probably not more than half that amount is actually used. The fact that the field in the telescope is necessarily much less than that of the unaided eye is the greatest objection to the use of telescope sights. It must be admitted, however, that with the target once in the field, the process of sighting the gun will be very much facilitated. No judgment is required; no fine eyesight; the gun is simply moved until the image of the target coincides with the cross-wires. It is held by many that the firing will be less rapid with these sights than with the ordinary bar sights, although others who have given the matter careful thought contend that it will not. When we consider that the element of doubt is totally removed, perhaps the latter are correct. At any rate we must always remember that our object is to hit, and not merely to get shots overboard, and if we make five hits out of five shots it is much better than making five hits out of seven shots in the same time.
In damp or misty weather the telescope is nearly useless. To provide against this, each one is fitted with a pair of finder sights. These are exact reproductions of the heads of the front and rear bar sights now in use, and are secured to the top of the telescope with their line of sight parallel to the optical axis. They are but 13".7 apart, which is less than half the distance that is customarily allowed between sights; they should never be used except in the above case, or when it is desired to pick up the target in rough weather preparatory to the finer sighting with the telescope.
If a piece of paper is held at the point at which the pupil of the eye is placed in sighting through one of these telescopes, a bright spot is shown on the paper about 0.2 inch in diameter, which is just about the diameter of the normal pupil. If any portion of the pupil is coincident with the bright spot, the image of the cross-wires and target are seen. A movement of the eye throughout the whole coincidence of the bright spot and the pupil does not disturb the relative position of the two images, provided the cross-wires have been properly placed. It is apparent, then, that the head can be moved through nearly 0.4 inch without interfering with the sighting, while with the bar sights the eye must be kept in line with the target, front and rear sights. This constitutes an appreciable advantage that lies with the telescope sight.
There is but one adjustment to be made with these sights, and that is to get the optical axis of the telescope parallel with the axis of the gun when the pointer is at zero.
It cannot now be stated how often this, adjustment will be necessary. It will depend largely on the care that is taken in handling and cleaning the instrument and its operating mechanism.
The parallel motion between the telescope and gun is assured if the distance between the center of the trunnions on the slide and the cross-shaft remains the same. Small variations in these distances caused by the racking of the turret will not affect the parallelism sensibly, although it will be necessary to readjust the telescope in such case. Recently at Indian Head an experimental 13-inch turret was struck by two shots from a 12-inch gun and one from a 10-inch gun without causing any apparent racking of the structure. There is, therefore, little ground for the fear that the sighting mechanism would be injured in battle from this cause.
The simplest way of restoring the gun and telescope to parallelism is by sighting through bore sights at some distant object, setting the drum to the zero reading, and adjusting the telescope so that the cross-wires cover the same object. A distance of not less than 2000 yards should be selected, if possible, in order to make the parallax inconsiderable.
In case the ship is not steady enough to use this method, fixed objects on board may be used. The gun should be leveled or brought to some other datum point, the telescope set to zero, and the distance between gun and telescope allowed for in the arrangement of the points to be sighted at. It is proposed to furnish an appliance by which this adjustment may be made simply and quickly.
So far as the meager returns tell us, these sights give good promise of greatly improving our gun practice.
In the 8-inch and 13-inch scores shown below the ship was rolling five degrees and pitching three degrees. The time between shots was less than has been attained heretofore with bar sights. In the lo-inch the time between shots was 2 min, 25 sec, which is about equal to that attained with bar sights, but the score is much the best that has hitherto been handed in with guns of that caliber. All of the scores are on vertical targets, with the size of the heavy shots drawn to scale.
Objection has been made to these sights on account of the care with which they must be handled. They are not as delicate as the sextant or the chronometer, and yet we consider these instruments indispensable to the ship and take the greatest pains that they shall fulfill their object. Every effort has been made to give the mounting the strength and simplicity that ship conditions call for, and yet it must be confessed that this makes another addition to the numerous delicate instruments now to be found on a modern man-of-war. But if we remember the prime cause for which the ship is built—a fact too often forgotten—and that is, to carry guns capable of striking blows at the enemy, we should not stop at any care or pains in providing the final link in the long chain that we have constructed to that end.