Now the telescopes are directed upon the object the distance of which is to be measured, and this object is marked at T in the diagram. It will be seen that the telescopes are, in fact, located at the extremities of a base-line A B, which may include the whole length of the vessel, or her entire breadth of beam. In the one case the two instruments would be located at the stern and on the forecastle, and in the other at opposite extremities of her bridge. Of course, the length of this base-line is known, and the distance A T is the range which is to be found out. Without going into the trigonometrical discussion involved, it will suffice to say that the distance A T depends upon the extent of the angle (A which is included between the lines of sight of the two telescopes which are directed upon the object.
If, then, one of these telescopes be moved from the position C to the position C', for example, it will be evident that the angle included bet ween the two positions of the telescope (C (') will be equal to the angle A TB, and also will be measured by so much of the arc E as is included between these two positions of the telescope. But the change in position of the telescope, as has already been described, causes a disturbance of balance in the electrical cir cuit ; and if the change in position of the telescope bears a relation to the range, as it does, then whatever measures the disturbance in the electrii.al circuit due to that change will equally measure the range; so that finally the range is shown by the extent of movement of the index-needle of the indicating instrument over its dial.
All that is done in practice is to station two observers at the two telescopes and cause them to direct their instruments upon the object. Then a third observer notes at once the range shown on the dial of the indicator. If the object moves, the two observers at the telescopes simply follow it with their instruments, and the needle of the indicator then moves as the range changes. Where the observers are separated by a considerable distance—as. for example, the entire length of a vessel—they may connnunicate with one another by an ingenious tele phonic arrangement which is provided. The telephone transmitter and receiver are connected directly to the telescopes, so as to partake of their motion, and are so supported that the instrument talked into comes directly in front of the mouth of the observer, while the instru ment at which he listens is held close to his ear. In this way one observer can tell to the other not only what object to look at.. but upon what part of an object to direct his sight— often a very important matter when the presence of several objects may create confusion, or when the target or some portion of it is more or less obscured by smoke, or when the observers are screened from one another by deck structures.
The indicating instrument may be placed in any convenient, position. and at any distance from the telescopes. There may be but one indicating instrument located, for example, at a given gull which is to be controlled. or any number of such instruments may be placed in the same circuit, when all of them will operate simultaneously to show the range.