When the gun is fired, the projectile breaks a wire of the first target and the circuit of which it forms a part. The electromagnets E and E' lose the power of attracting their armatures, the inducing current is broken, and a spark passes from the point of the needle to the cylinder, and in its passage through the paper leaves a black spot which indicates the position of the needle at that instant. The electro-magnet E' having lost the power of attracting its armature, it is brought in contact with the stop q', thereby closing the circuit of the second target. The circuit embraces the helices of the electro-magnets E' and E, which are thus made active. E' attracting its armature, breaks the connection with the third target, and E closes the current through the Ruhmkorff coil. When the ball breaks the second target, the electro-magnets E and E' cease to attract their armatures, a second spark leaves its trace on the paper on the cylinder, and the circuit through E' being completed, E again becomes active, and the inducing current is closed ready for the ball when it reaches the third target: in like manner are the currents completed till all the targets are broken. The time which elapses during the passage of the ball between any two targets is given by the angular distance between the two corresponding spots on the paper, and can be read off to thousandths and ten thousandths of a second.

The time of a single revolution of the needle is determined by placing in the circuit of the Daniels battery a pendulum beating seconds, and so arranged as to break the circuit at each oscillation. Each beat of the pendulum will be thus recorded on the paper by a spot, and if the spark passes through at the same spot each time, the needle will of course make a revolution in exactly one second. This has been accomplished by oft-repeated trials and by varying the length of the pendulum, which has a screw under the bulb intended for the purpose.

The mean time of a revolution may be determined with as great accuracy as desired by counting the number of revolutions made in a given time, as determined by an accurate stop-watch. The uniformity of the motion of the needle is verified by placing in the circuit of the Daniels battery the pendulum of an astronomical clock. The isochronal oscillations of the pendulum will be recorded on the paper, from which it can be readily seen whether the motion of the needle is uniform. Actual experience has shown that this is attained by this instrument. The targets have been put as close together as fifteen feet. The instrument just described was purchased by the French Artillery committee for the purpose of making experiments on the flight of projectiles.

There yet remains to be noticed another chronoscope, the most beautiful and ingenious in design of any that have preceded it, and one which gives results of most wonderful accu

racy: I refer to that of Captain Schultz, of the French Artillery. It is peculiar in its construction, there being two points in which it differs materially from all others. To measure the minute fractions of a second of time necessary for the purpose for which ballistic chronoscopes are required, instead of using a pendulum making at most not more than three oscillations in a second, Captain Schultz has selected for this purpose a tuning-fork, which makes about five hundred vibrations in a second; and to eliminate as far as possible all sources of error, Captain Schultz rejects all intervening rheotomic arrangements which may cause in the rupture of the inducing current delays, of which it is difficult to form an exact idea, and constructs the targets in such a manner that the rupture of one shall complete the circuit through the next, so that there shall be required only a single battery and one Ruhmkorff coil for all the targets to make as many observations as may be desired on the flight of the same projectile. The accuracy of this instrument in no way depends upon a uniform motion being communicated to a revolving cylinder, as in that just described, but is based upon the fact that the vibrations of the same tuning-fork are isochronal. This is the case, to a very close approximation, when the length of the vibrations vary only within moderate limits, and is strictly correct when the amplitude of the vibrations remains constant.

The instrument consists essentially of a registering apparatus in which the spark from a Ruhmkorff coil is used: the tuning-fork and electro-magnet for making it vibrate, including the interrupter; two batteries; and the required number of targets, which are of a special construction.

The registering apparatus consists of a silver-plated brass cylinder, A, on a horizontal axis, X, which is turned by a system of clock-work put in motion, by a weight acting on the wheel, W; the motion is regulated by the fan, F. The cylinder, besides having a motion of rotation, has also one of translation in the direction of its axis, by means of the screw, g, beneath the cylinder, which is thrown in or out of gear by a lever, h. By loosing the thumb-screw, s, the clock-work is disengaged from the shaft, X, and the cylinder may be turned by hand by the wheel, T. A stop-lever, L, commands the movement of the clock-work. The cylinder, A, is covered with a uniform light coat of lampblack from a smoking lamp. The tuning-fork, K, is fastened to the frame between the electro-magnets E, E', which, acting on two armatures fixed to the two branches of the fork, cause it to vibrate. The left branch has a quill pen fastened to it by a screw, and a small eccentric serves to regulate the contact with the cylinder, so as to make a very fine trace in the lampblack, The tuning-fork is gilded to prevent oxidation.

The interrupter, represented on p. 111, consists of a steel spring, a, with a platina point, b, projecting down into a

cup containing mercury. Two electro-magnets, E, are placed over and near a piece of soft iron screwed on top of the spring. Two small movable weights, s, s', are placed on the spring to regulate its vibrations. The interrupter is placed in the circuit of the battery which excites the electromagnets E and E' of the tuning-fork, by joining the conductors to the press screws, J J'. When the circuit is closed, by turning the lever, p, the electro-magnets EE of the interrupter raise the spring, a, so that the platina point does not touch the mercury this breaks the current, and the spring, released by the electro-magnet, flies back, thrusting the platina point into the mercury, which closes the circuit and again renders the electro-magnet active: the operation of closing and breaking the circuit is thus continued in quick succession, depending upon the vibrations of the steel spring. By varying the position of the weights 8, 88', the spring is made to vibrate in unison with the tuning-fork: and the vibrations of the latter are kept of the same amplitude by the action of the electro-magnets E E'. The cup, C, is supported on a stem screwed into the stand: by turning the cup to the right or left it is lowered or raised. This also varies the time of the vibration of the spring. To protect the mercury from oxidation, its surface is covered with a little alcohol in the cup.

The induced current from the Ruhmkorff coil is conducted by insulated wires, one pole to the plated cylinder A, and the other through the wires S S, through two glass tubes, very near to the surface of the cylinder at Q, but not touching it. When the inducing current of the Ruhmkorff coil is broken, an induced spark flies from this point of the wire to the cylinder, and burning off the lampblack, leaves the silver surface exposed, which is readily seen, and marks the position of the cylinder at the time the spark occurred. A micrometer, M, is used to measure with accuracy the position of the spot made by the spark in reference to the trace formed by the vibrations of the tuning-fork. It is readily detached from the instrument by loosening the screw, K.

There are two batteries required for the use of this instrument: one to make the tuning-fork vibrate, and the other to

produce the sparks. The first communicates with the interrupter through the break, p, and the second connects with the pendulum, or the target, as the lever, l, is turned to the right or the left.

The targets may be of any number desired. They are constructed with wires running across a wooden frame, so that the projectile cannot pass through without breaking a wire, and also so arranged, by means of springs or weights, that the rupture of any one of the cross-wires of the target will complete the circuit through the next target, and thus to the last, so that a single battery and one Ruhmkorff coil will produce a spark when each target is broken, no matter how numerous they may be.

It is not necessary to give a minute description of the details of the construction of these targets. Several different ways of effecting the desired object have been devised, and that adopted in this country is not like the plan described and used by the inventor in France.

The cylinder, A, being covered with a slight coat of lampblack, and being set in motion by winding up the weight of the clock-work, by the crank, Z, the pen on the end of the tuningfork being in a state of rest, will describe a helix on the surface of the cylinder, extending from one end to the other. If the cylinder be brought back to its first position and again started with the tuning-fork vibrating, the point of the pen will describe a sinuous line, perfectly regular, thus:

If, then, the time of vibration of the tuning-fork be a known, we shall have traced on the cylinder a scale of time of exactly equal parts, the unit of which is very small, being the duration of a single vibration. To use this scale of time in the determination of the velocity of projectiles between two targets placed at any part of the trajectory, it is only necessary to record by the side of this scale of time the precise instant when the projectile passes through the different targets, and this is done as described before, by the Ruhmkorff coil. The number of entire vibrations are counted, and the fractions of vibrations are estimated by the eye in cases where the greatest accuracy is not required. The error thus made will not exceed one twenty-five hundredth part of a second. Where still greater exactness is required, the micrometer is used, by which the exact fraction of a vibration can be accurately measured.

с

To determine the time of a single vibration of the tuningfork, a pendulum, R, beating half seconds, is put in connection with the Ruhmkorff coil, and so arranged by means of a spring, q, that at each double vibration it breaks the inducing current, and this gives a spark on the cylinder. By this means, along the side of the curve a series of sparks are made marking seconds of time, and the number and fractions of vibrations of the tuning-fork in a second of time can

be readily counted. As the cylinder will revolve for thirty seconds, if the whole time be divided by the number of vibrations between the extreme sparks, the time of a single vibration will be determined with very great accuracy.

To determine the velocity of a projectile with this instrument, it is established on a strong table resting on a firm foundation, independent of the building in which it is placed; the targets are disposed at such points of the trajectory as may be desired, and the connections with the batteries and instrument made. The silver cylinder is covered with a light coat of lampblack, as already described. On closing the circuit through the interrupter, by turning the lever, p, to the position represented in the drawing, if the weights on the interrupter be properly placed, the tuning-fork will commence to vibrate; if it do not vibrate, the position of the weights must be changed.

The determination of the proper positions of these weights, at first, may require many trials and some exercise of patience, as is known from experience in putting up the first instrument of the kind in this country, unassisted by special instructions to serve as a guide, and the weights being placed far from their correct positions; but when these are once determined, a few turns of the cup containing mercury to the right or left will suffice generally to start the tuning-fork to vibrating. If the mark left on the cylinder be fine, regular, and uniform in extent, turn down the lever, h, raise the lever, l, and the clock-work will set the cylinder in motion, and the gun may be fired. The projectile breaking the targets in succession, the exact time of each rupture will be recorded by the sparks on the cylinder, in units of a single vibration of the tuning-fork, and can be readily reduced to seconds and fractions of a second. As many as fifteen observations may be made before the lampblack on the cylinder will require to be renewed, which is readily done.

The accuracy with which this instrument notes the velocity of projectiles cannot fail to strike every one who witnesses it with admiration and astonishment. The inventor

says he has measured, with a very close approximation, a portion of time the total duration of which did not exceed the one five thousandth part of a second, and that he has measured the time required for a ball to pass through dif ferent lengths of the bore of a cannon, determining its velocity at the different points. The writer had just completed his preparations to repeat this experiment when the order directing a change in his station prevented him from pursuing further this very interesting subject of inquiry. That his trials would have proved highly satisfactory can hardly be doubted, judging from the results of experiments made with the same instrument and targets on the time of flight of a musketball through the space of one foot-two consecutive shots un

VOL. IV.-3