The accompanying cut illustrates a telemeter which was exhibited at the Paris Exhibition of Electricity, and which is particularly interesting from the fact that it is the first apparatus of this kind. It will be remembered that the object of a telemeter is to make known at any moment whatever the distance of a movable object, and that, too, by a direct reading and without any calculation. In Mr. Siemens' apparatus the problem is solved in the following manner:

The movable object (very often a vessel) is sighted from two different stations--two points of the coast, for example--by two different observers. The sighting is done with two telescopes, A1 and A2, which the observers revolve around a vertical axis by means of two winches, K1 and K2, that gear with two trains of clockwork. There is thus constantly formed a large triangle, having for its apices the movable point sighted and the vertical axes, A1 and A2, of the two telescopes. On another hand, at a point situated between the two telescopes, there is a table, T T, that carries two alidades, a1v1, and a2v2, movable around their vertical axes, a1 and a2. The line, a1 a2, that joins these axes is parallel with that which joins the axes of the two telescopes; and the alidades are connected electrically with the telescopes by a system such that each alidade always moves parallel with the telescope that corresponds to it. It follows from this that the small triangle that has for apices, a1 a2, and the crossing point of the two alidades will always be like the large triangle formed by the line that joins the telescopes and the two lines of vision.

If, then, we know the ratio of a1, a2 to A1 A2, it will suffice to measure on one of the alidades the distance from its axis to the point of crossing in order to know the distance from the movable object to the axis of the corresponding telescope. If the table, T T, be covered with a chart giving the space over which the ship is moving, so that a1 and a2 shall coincide with the points which A1 and A2 represent, the crossing of the threads of the alidades will permit of following on the chart all the ship's movements. In this way there maybe had a powerful auxiliary in coast defence; for all the points at which torpedoes have been sunk may be marked on the chart, and, as soon as the operator at the table finds, by the motion of the alidades, that the ship under observation is directly over a torpedo, he will be able to fire the latter and blow the enemy up. During this time the two observers at A1 and A2 have only to keep their telescopes directed upon the vessel that it has been agreed upon to watch.

SIEMENS' ELECTRIC TELEMETER

In order to obtain a parallelism between the motion of the alidades and that of the corresponding telescopes, the winch of each of the latter, while putting its instrument in motion, also sets in motion a Siemens double-T armature electromagnetic machine. One of the wires of the armature of this machine, connected to the frame, is always in communication with the ground at E1 (if we consider, for example, the telescope to the left), and the other ends in a spring that alternately touches two contacts. One of these contacts communicates with the wire, L1 and the other with the wire, L3, so that, when the machine is revolving, the currents are sent alternately into L1 and L3. These two latter wires end in a system of electro magnets, M1, provided with a polarized armature. The motions of the latter act, through an anchor escapement, upon a system of wheels. An axle, set in motion by the latter, revolves one way or the other, according to the direction of the telescope's motions. This axle is provided with an endless screw that gears with a toothed sector, and the latter controls the rotatory axis of the alidade.

The elements of the toothed wheels and the number of revolutions of the armature for a given displacement of the telescope being properly calculated, it will be seen that the alidade will be able to follow all the movements of the latter.

When it is desired to place one of the telescopes in a given position (its position of zero, for example), without acting on the alidade, it may be done by acting directly on the telescope itself without the intermedium of the winch. For such purpose it is necessary to interrupt communication with the mechanism by pressing on the button, q. If the telescope be turned to one side or the other of its normal position, in making it describe an angle of 90°, it will abut against stops, and these two positions will permit of determining the direction of the base.

The alidades themselves are provided with a button which disengages the toothed sector from the endless screw, and permits of their being turned to a mark made on the table. A regulating screw permits of this operation being performed very accurately. In what precedes, we have supposed a case in which the movable point is viewed by two observers, and in which the table, T T, is stationed at a place distant from them. In certain cases only two stations are employed. One of the telescopes is then placed over its alidade and moves with it; and the apparatus thus comprehends only a system of synchronous movements.

This telemeter was one of the first that was tried in our military ports, and gave therein most satisfactory results. The maneuver of the winch, however, requires a certain amount of stress, and in order that the sending of the currents shall be regular, the operator must turn it very uniformly. This is a slight difficulty that has led to the use of piles, instead of the magneto-electric machine, in the apparatus employed in France. With such substitution there is need of nothing more than a movable contact that requires no exertion, and that may be guided by the telescope itself.--La Lumière Electrique.