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. [Illustration: 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.