An instrument for determining the position of objects which are to be
fired at from forts. It is designed for use from forts situated on the
water.

Fiske’s position finder may be thus generally described. On a chart the
channel is divided into squares, and the position finder determines the
square in which a vessel lies. For each square the direction and
elevation of the guns is calculated beforehand. The enemy can therefore
be continuously located and fired at, although from smoke or other cause
the object may be quite invisible to the gunner.

It comprises two telescopes situated at distant extremities of as long a
base line as is obtainable. These telescopes are kept directed upon the
object by two observers simultaneously. The observers are in constant
telephonic communication. As each telescope moves, it carries a contact
over an arc of conducting material. Below each telescope is an arm also
moving over an arc of conducting material. These arcs enter into a
Wheatstone bridge and are so connected that when the arm and the distant
telescope are at the same angle or parallel a balance is obtained. Thus
each observer has the power of establishing a balance. A chart is
provided for each of them, and over it the arm connected with the
distant telescope and an arm or indicator attached to the telescope at
that station move so that as long as both telescopes point at the object
and each observer maintains the electric balance, the intersection of
the arms shows the position on the chart.

The Position Finder is a simplification and amplification of the Range
Finder, q. v. In practice the observers may be placed far from the
forts, and may telephone their observations thereto. It has been found
accurate within one-third of one per cent.