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 p. 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' E2 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 q' 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 2, 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.

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.