POSITION OF BULLET IN FLIGHT

335

left-hand spiral of similar inclination, since by this effect of the rotation of the earth 6 inches were in the one case added to, and in the other subtracted from, the deviation due to drift. That the pressure of the air upon the bullet is very considerable at such high speeds as are attained by modern rifles admits of no dispute. It has been said that at a speed of 2,200 f.s. the resistance of the air amounts to 28 lb. to the square inch, and that, assuming a bullet of the size of the Lee-Metford moving at this speed through the air, it would meet with as much resistance as if it were moving in a vacuum with no air to oppose it, but were dragging with it a weight of 24 lb. It is the pressure of the air upon it which causes the bullet to keep itself constantly pointed in the direction in which it is moving; if it were moving in a vacuum, it would have no tendency to point in any other than its original direction, and after a long flight would fall with its nose well up in the air, keeping its position parallel to that which it had when it left the muzzle of the piece. This fact, that the axis of the bullet really follows very closely the curve of the trajectory, used to be doubted, and is even now not quite as well known as it should be, probably because at very low velocities the phenomenon is not noticeable. The writer has seen diagrams entirely misleading upon this point, in books written by those who had only a theoretical knowledge of the subject. At whatever distance from the muzzle the bullet may strike, it will be found in perforating such a body as a wooden target to make a round hole, if it has had sufficient spin given to it in the first place. This has been practically demonstrated with the rifle over and over again at distances of 2,000 yards and more, and with large shot fired from big guns it requires no elaborate proof, for it is obvious to the eye.

The group which a first-rate rifle will make is, up to 1,100 yards at all events, not far from proportionate to the distance of the target under equal conditions. A series of shots may often be placed within an angle of 3', i.e. 3 inches at 100 yards, and not infrequently within the same measurement, i.e. 30 inches, at 1,000 yards, although there is a distinct tendency for the shots which deviate from the general

line of the group to wander further and further away from it. One curious phenomenon which may be mentioned here is that if a series of shots be fired through several screens of thin paper interposed in the line of the trajectory, it will be found that hardly any one shot will maintain on all the screens its position relative to the others, even in the calmest weather. In the central part of the group, which represents the normal shooting, the shot which is highest in the first screen will often not be so in subsequent ones, and so on. The shots really change places in a way not to be accounted for by any atmospheric movement. When we come to those more wandering shots which, edging away in any direction from the centre of the group, so often spoil a series of bull'seyes, both at short range and at long, they seem to have some eccentricity of flight which carries them out from the group, and they seem to wander further from it at the longer ranges. Whether these abnormal shots are produced by some irregularity of ignition of the powder, or by some casual deposit of the fouling in the barrel, causing unusual friction, or, it may be, setting up some odd movement of the muzzle at the moment of firing, is not clear; but while the general grouping of the central shots with smokeless powder in modern military rifles is closer than it was with the best of the old military breech-loaders firing black powder, the outer shots seem often to fly more widely and more wildly.

To ascertain the regularity of different loads, as well as the actual velocity which they give to the bullet, many devices have been and are used. It is not possible here to describe them at any length. The earliest machine for the purpose, that used, for instance, by Robins, Count Rumford, and Hutton, was a pendulum; the measurement of its movement, on receiving the impact of the shot, enables the velocity to be calculated, the weights both of the pendulum and of the bullet being known. This instrument in its old form was not very satisfactory, the relation of the point of impact to the centre of gravity having to be taken into account. The writer has one similar to that used by Mr. Metford in his experiments, which is hung by four wires in front and the same number behind, a length of some 2 feet

THE BALLISTIC PENDULUM

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6 inches being given to the pendulum bob, and the wires spread laterally at the points of suspension, so that it can only swing in one direction. The attachment of the wires to the pendulum and to the sockets from which it is hung is by bearings having knife-edges. This reduces friction and makes the movement of the pendulum independent of the precise point at which the face of it is struck. To get true results with a pendulum it is necessary that the whole of the fragments of the shot should be caught, and this is effected. by fitting a steel shield 8 inches in diameter in the head of the pendulum and placing in front of it a wooden block some 2 inches thick. The bullets pass through this and break up on the steel shield, and their fragments cannot escape. The weight of the wooden block, which has to be renewed after a few score of shots, is taken into account, and the weight of the pendulum is maintained by placing upon a little shelf attached to it a number of bullets of the kind about to be used, and removing one for each one that is fired into the pendulum. A sliding index pushed back by an adjustable screw at the tail of the pendulum gives the measurement of the horizontal swing from which the velocity is calculated.

The

It is, of course, really the height to which the pendulum is raised that forms the basis of the calculation. length of suspension, ascertained precisely from the time of swing, is about 100 inches, and the pendulum weighs when ready for firing about 140 lb. Its movement is easily read to of an inch, which with the 256 bullet of 156 grains represents about 1 foot of velocity. The disadvantage of the pendulum is that the weight of the bullet requires to be known with considerable accuracy. Chronographs are therefore more commonly used, which depend upon breaking and making an electric circuit. In the Boulengé chronograph used by Bashforth in his experiments, and improved by Major Holden, the passage of the bullet through two wire screens placed at some little distance apart first releases a steel bar suspended by one end by magnetic attraction; the breakage of the circuit at the second screen releases another bar, which falls on a trigger, causing a knife to make a mark on the falling bar first released. The length

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of the bar representing the amount of its fall is then measured by a suitable scale giving the equivalent in velocity of the time taken in the passage between the screens.

Another class of chronograph, such as that invented by Professor Jervis Smith, depends upon the movement of a cylinder or a piece of smoked glass, on which a tuning-fork marks a scale of time, the instants of breakage of the circuit at the screens being marked against the scale. Dr. Crehore and Dr. Squier seem to have hit on a new and successful method of obtaining a record of minute intervals of time by photography. The sensitive plate moving at a known velocity, a ray of light is admitted on the first breakage of the current, and extinguished on the second. This is effected by passing the ray through prisms, and then through a liquid which, under the influence of an electric current, polarises the field. The method is fully described in Arms and Explosives,' September 1895.

The screens used with the Boulengé chronograph are commonly placed 180 feet apart. Sometimes a wire close to the muzzle of the rifle is substituted for the first screen. The mean velocity of the shot between the screens is usually taken as being equivalent (as it very nearly is) to the velocity at a point midway between them.

A very old means of ascertaining velocity was by rotating two discs fixed at a short distance apart upon a rod, and measuring the amount of the rotation during its passage between the first disc and the second. This was unsatisfactory, as it postulated a very accurate knowledge of the direction of the bullet's flight, and, though he has seen it described in books, the writer has never heard of it being actually used.

The advantage which electric chronographs have over the pendulum is that the weight of the bullet does not require to be accurately known, since the time of flight over a given distance is measured. They are equally available for small arms or for cannon. They are, however, more costly and less simple, and perhaps not so well adapted for very occasional

use.

CLEANING-RUST-ITS CAUSES AND PREVENTION-CARE OF STOCK AND

LOCK-PULL-OFF-MISS-FIRES

THE incumbrances of the careful shooter are increased by his having to use constantly two sight protectors, one to cover the back sight, and the other the fore sight. The latter is usually a piece of tubing which fits over the muzzle of the rifle, and has a high recess formed in it, which encases the fore sight. Sometimes it is secured by a bayonet catch, and the box covering the sight is closed by a little lid. Sight protectors of this kind usually have the end completely covered so that dirt and sand are prevented from entering the muzzle. It is not many years ago that sight protectors were occasionally made with a piece of brass tubing to fit into the bore at the muzzle, in order, presumably, to give them a firmer hold, but this abomination, which was very likely to injure the internal surface of the barrel, seems now to have been generally discontinued. The fore sight protector exists mainly to secure the sight against damage from a fall or a blow. We have already spoken of other methods of protecting the sights, and it may be hoped that before long the detached protector may become, so far as the English military weapon is concerned, a thing of the past. The function of the back sight protector, as used by the marksman, is chiefly to prevent any colouring matter put on the sight from being rubbed off; it is not recognised as suitable for military service. The arrangement should be such that the back sight may be covered when the slide is set at any desired elevation, without being necessarily lowered to the bottom.

There are several accessories which the target shooter