electro-magnet like that of Captain Navez. The currents from the two batteries are conducted through the targets, thence to two Ruhmkorff coils, and the induced currents carried one pole to the pendulum, and the other to a brass plate behind the vertical graduated arc. The projectile, passing out of the bore of the gun, breaks a wire in front of the muzzle, and the current holding up the pendulum, which begins at once to fall, the breaking of the target breaks the inducing current of the first Ruhmkorff coil, and a spark of electricity darts from a projecting point on the pendulum through a piece of paper held against the brass plate, which forms the opposite pole of the induced current; the projectile, in passing through the second target, breaks the inducing current of the second Ruhmkorff coil, and sends a second spark in the same manner through the paper. The time of oscillation of the pendulum being determined, it can be calculated what time elapsed while the projectile was passing over the distance between the two targets, indicated by the angular distance between the two sparks. The paper is prepared chemically, so that the spark, in passing through it, either burns a black hole or effects a chemical change, and leaves a mark which can be readily distinguished. Paper saturated in a solution of the ferro-cyanide of potassium is preferred for this purpose. The advantages of this instrument over those which preceded it are manifest. The pendulum is most delicately mounted, and is swinging freely through the air, when it notes the passage of the projectile. There is no error from the electro-magnet not losing its magnetic force instantly. No disjunctor is used. The instant at which the projectile passes the two targets is indicated by an immaterial index. The objections to it are, first, that the pendulum, as arranged by Captain Vignotti, attains only a slow velocity before the wires of the target are broken, and a slight error in reading the angles would make a great error in the velocity of the shot. It is also known that the spark in its passage does not always travel the shortest distance from the point to the plate; but when the point is made to move very close to the paper, this last error is not, under any circumstance, considerable. The first objection was entirely removed by a simple improvement made to the instrument in this country. The gun is discharged by means of a small weight or drop falling on a percussion-cap. This drop, in its descent, breaks at any desired point the current of the electromagnet which holds up the pendulum, and the time of flight of the projectile is then noted by the pendulum when it is moving most rapidly, and any error that may be made in reading off the angle will have the least effect on the correctness of the result. This instrument, thus arranged with a large graduated arc, and a pendulum vibrating quickly, with a long arm to reach to the graduations on the arc, has given highly satisfactory results.

Major Martin de Brettes, of the French Artillery, and professor in the artillery school of the Imperial Guard, has recently proposed, and Mr. Hardy has constructed, a chronograph, which has given more accurate measurements of time than could possibly be obtained by any of the instruments already mentioned. It is represented in the drawing below. It consists of a registering apparatus, using for this purpose the spark from a Ruhmkorff inductive coil-an arrangement for breaking and closing the galvanic current as the projectile breaks the wires of the target-two galvanic batteries-the required number of targets, according to the different observations to be taken. A Bunsen battery is preferred to produce the inducing current of the Ruhmkorff coil, and a Daniels battery for the current which passes through the different wire targets.

The registering apparatus consists essentially of a vertical copper cylinder, A, around which is stretched a strip of prepared paper. The bent needle, B, its platina point turned normal to the surface of the cylinder, moves around and very near to it, but not touching it. The needle receives its motion from a vertical spindle in the axis of the cylinder, and is turned by a system of clock-work, constructed with great accuracy, with helicoidal toothed wheels, and without escapement, to obtain a continuous motion, which is rendered uniform by the inertia of the revolving pendulum, C. The cylinder is so arranged that it can be moved very slowly in the direction of its axis by a special mechanism placed within it, and operated either by hand, or automatically, by means of an electro-magnet acting on a stop, so that the sparks from the point of the needle, instead of forming a circle around the cylinder, will be found in a helix, and experiments can be made on the flight of projectiles lasting for thirty seconds. The cylinder can be turned around on its axis by hand, for the convenience of the observer in reading off the results. The surface of the cylinder is graduated into a thousand equal parts; and the needle, revolving around it once in a second, each of the divisions will correspond to the thousandth part of a second. The needle is insulated from the cylinder, and each communicates with one of the poles of the Ruhmkorff coil by the pressscrews po and po'. The Ruhmkorff coil, H, is connected with the Bunsen battery, P, through the armature of the electromagnet, E.

It is not a matter of indifference how the connections are made, in order to obtain a spark best suited for the purpose. When the outer pole of the induced current is connected with the needle, and the inducing current enters so as to make this pole the negative pole, the spark travels more directly, and gives a better mark than when the positive pole is joined to it.

The rheotomic arrangement consists of a series of electromagnets with armatures, E E' E' E'. That marked E closes and breaks, by the motion of its armature, the current passing through the Ruhmkorff coil. Those marked E' E' E' are used to pass the current through the different targets in succession, so that a single battery may answer for all the targets. The armature of the electro-magnet E is kept from the magnet by a slight spring, which is overcome by the force of the magnet when the current is completed, and the armature is brought in contact with the stop g'; this completes the current of the Bunsen battery, and a spark is produced as often as the armature touches or is separated from the stop g'. The electro-magnets E E' E' are arranged so that their armatures 'a 'a 'a touch their stops q' q' q' when their electro-magnets are not excited, and this contact is broken when the circuit is closed.

The different parts of this apparatus are connected as follows: One end of the helix of the electro-magnet E is carried to the press-screw d, where it is joined to a conductor running to one pole of the Daniels battery, D. The other end is carried to the press-screw, 'd, and thence to the helix of the electro-magnet E', which connects it with the screw C.' The stop q' of the armature 'a of the electro-magnet E' is joined by two wires 'd'd to the armature and to the helix of the electro-magnet E'; the other end of the helix E' communicates with the battery through the screw C. The connection of the electro-magnet E' with the third E' is made in the same manner. The stop of the last electro-magnet E connects directly with the battery through the screw C.

A single Daniels battery is thus sufficient, no matter how many targets are used; for, the instant that the projectile breaks the wire of one target, the arrangement of the electromagnets is such as to complete the circuit through the next, and so on in succession to the last. A strip of paper steeped in a solution of the ferro-cyanide of potassium for several hours, and dried, is stretched smoothly on the cylinder A, which has a slit to receive one end of the paper for this purpose. The pendulum is started, and in a few moments takes up a uniform motion. The connections of the batteries are made as indicated in the figure. The current of the Daniels battery excites the magnet E, which, attracting its armature, a, brings it in contact with the stop q, and thus closes the circuit of the Bunsen battery. It also renders the electro-magnet E' active, which, attracting its armature, breaks the connection with the second target at q'. The circuits through the targets Nos. 3 and 4 passing through this stop will be broken necessarily, and there will be only two circuits closed, the inducing circuit and that through the first target.

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