ELECTRO-BALLISTIC CHRONOSCOPES.

"Il serait à désirer qu'on s'occupât d'un moyen facile et exact de calculer la vitesse initiale."-LOUIS NAPOLEON BONAPARTE.

THE want expressed in the above quotation from the "Manuel d'Artillerie" of the Emperor Napoleon, who, to the high quali ties of a great sovereign and skilful general, adds the unusual accomplishment of an able and experienced artillerist, is one that has long been felt by artillerists. The ballistic pendulum, first proposed and used by Robins and improved by Hutton, and the gun-pendulum of Count Rumford, were at this time, and until within the last few years, the only reliable instruments for determining the initial velocity of projectiles. The information obtained by these means has been of great value in solving questions of the utmost importance in artillery and gunnery. For the last thirty years these instruments have undergone no modification or improvements, and besides being expensive to establish and difficult and troublesome to adapt to the varied wants of the service, there are many points of interest in connection with the flight of projectiles, to the determination of which they are not applicable: for instance, the velocity of projectiles fired at high angles, as mortar-shells; the terminal velocity of shot, of more particular importance in the case of shrapnel shells; and the velocity of a ball at two or more points of its trajectory-data for the solution of that nice question in ballistics, the influence of the air in impeding the motion of projectiles.

Not long after the subtle fluid, which was always looked upon only with feelings of awe and dread, had been tamed by man, and the means discovered whereby it could be made the quick messenger of his thoughts and wishes, the idea was conceived of making its true obedience and lightning speed subservient to the wants of the artillerist, by recording for him the rate of travel of the cannon-ball at different parts of its

course.

To Wheatstone is the credit due of publishing first, in 1840, a description of an electro-chronoscope for determining the initial velocity of projectiles. According to this description, his instrument was to consist of a system of clock-work, which was put in motion or was stopped according as an electro-magnet attracted a piece of soft iron or released it when the current passing through the helix of the magnet was broken. The time of flight of the ball was measured by the length of the arc passed over by the index-hand. The target was so arranged, that the least motion given to it by the ball striking it released a small metal spring, and established a permanent contact be

tween the spring and another piece of metal on the target-frame, again completing the circuit. One pole of a small battery was attached to one end of the helix of the electro-magnet; from the other pole were led two wires, one of them to the target and the other to a wire passing in front of the muzzle of the gun. To the other end of the helix of the electro-magnet were attached two wires, one of which was connected with the spring at the target, and the second to the other extremity of the wire passing in front of the gun. An unbroken circuit was thus formed, the wire in front of the muzzle of the gun being in it. When the gun was fired the ball severed this wire and broke the circuit, which was re-established by the ball striking the target. The time between the breaking of the circuit and its being re-formed corresponded to the flight of the projectile, and was indicated by the index-hand. This first conception of an electro-ballistic chronoscope, imperfect, of course, as it was, and bearing as little resemblance to the perfected instrument as the germ usually does to the full-developed flower, soon gave place to others, one of which was furnished by Wheatstone, in 1843, to Captain Konstantinoff, of the Artillery of the Guard of the Emperor Nicholas, and under his direction was put in operation.

In this country the pioneer in this field of invention was Professor Henry, then of Princeton College, now the Secretary of the Smithsonian Institution, whose elaborate investigations of the phenomena of electrical induction have given him so widespread a reputation, and to him belongs the credit of first proposing the use of the spark of an induced current of electricity to record the passage of the ball through a series of wire targets, for the purpose of determining the different velocities of the projectile-a long stride in advance of all others in this branch. of invention. His attention was first called to the importance of a more convenient and accurate means than was then known of determining the velocity of projectiles, by Commodore Stockton; and as he had for some time been engaged in a series of experiments on electro-dynamic induction, and in developing the principle of what is now known as the Ruhmkorff coil, and having succeeded in obtaining a spark of sufficient intensity to leave on prepared paper a distinctly visible mark, it occurred to him to use in the construction of his apparatus this new princi-. ple, which is now adopted in all of the best instruments for this purpose. In the proceedings of the American Philosophical Society, held in Philadelphia, May, 1843, is found the descrip tion of the instrument proposed by Professor Henry. It con sisted of a metallic cylinder revolving on a vertical axis with a uniform motion communicated by a system of clock-work; two or more target screens made of wires so placed that the ball in its flight would break in succession the wires of the targets, and interrupt the currents of electricity passing through them.

"A part of the long wire which leads to the screen is coiled around a bundle of soft iron-wire; and over this is coiled another long wire so as to produce an intense secondary current, on the principle of the common coil-machine. One extremity of the secondary circuit is connected with the axis of the cylinder, and the other is made to terminate almost in contact with the revolving surface, which in this modification of the instrument is surrounded by a ruled or graduated paper. It is obvious that the secondary current, which is induced by the interruption of the primary circuit, will pierce or mark the paper band at the moment of the screen being broken. There is no difficulty in effecting such a current of sufficient intensity to mark the paper, since Professor Henry, in some of his experiments on induction, has developed one which gave a spark between a point and a surface of nearly a fourth of an inch in length. The terminal points of the wires from the two screens may be placed very near each other in the same horizontal plane; if, then, the cylinder revolving horizontally has at the same time a slow ascending motion, the relative position of the dots on the paper will give the number of whole turns and parts of a turn made by the cylinder while the ball was passing between the two screens. In the same way the terminal points of wires from a number of different pairs of screens may be made to impress their marks on the surface of the same cylinder, and the velocity of the ball at the different points of its path may in this way be determined by a single experiment."

Owing to some misunderstanding about the construction of the clock-work machinery, the expense of which Professor Henry expected would be borne by the Government, this invention, blocked out in its essential parts more than twenty years ago, has been permitted to slumber until its details, beautifully elaborated, have been filled in, and the instrument, successfully set up in France, it has at length been placed in the hands of a skilful workman in this country for execution-not the only instance in which an American conception has failed to be developed till, taken up abroad, it has received the stamp of foreign approval and paternity.

The name most closely identified with the subject of electro-ballistic instruments is that of Captain Navez, of the Belgian Artillery. To him is due the credit of having been the first to construct and put into practical operation a chronoscope which has given good results, and is now extensively used in several different countries. It was first used in 1849, at Brasschaet, since which time it has undergone some modifications and improvements. It consists essentially of three separate and dis tinct parts:

1st. The pendulum, with its stand and magnet.