3 
side of the transverse wire, has the same length of conductor. Hence no current 
passes through the transverse wire and the needle of the galvanometer is at zero. 
If one of the needles be deflected the segments of the arcs included in the circuit 
are no longer equal, the equilibrium is disturbed, and the galvanometer reveals 
the passage of a current through the wire pp. 
Inversely, if the needles were in the first case at a certain angle, one to the 
other, and were moved till they were parallel, the current in the galvanometer 
would diminish and would be nil as soon as parallelism was effected. 
In all Captain Fiske’s instruments it is assumed that the electromotive force of 
the source of electricity is constant, likewise its interior resistance and that of 
the circuits. With the accumulators he employs that hypothesis is practically 
exact for a period of at least 24 hours, as has been shewn by experiment. As 
none of the instruments are worked continuously, that period suffices for a large 
number of observations without re-charging the accumulators. 
II. Telemeters. 
The two successive instruments of this character were called the cc Slide Kange- 
Finder ” and the “ Automatic Kange-Finder.” In principle both these instruments 
consist of two telescopes mounted at the two extremities of a known base upon a 
circuit, arranged, as already explained, as a Wheatstone’s Bridge. Each of these 
telescopes turns upon a pivot above a horizontal plate, and is movable in the 
vertical plane to admit of being pointed in any direction. Upon the horizontal 
plate and round the same pivot turns a metallic needle, of which one extremity 
moves upon the conducting arc included in the circuit, and which follows the 
movement of the telescope in the same vertical plane. When the two needles, 
and in consequence the vertical planes of the two telescopes, are parallel, the gal¬ 
vanometer is at zero. 
If thenT(I%. 3) be the object and 
AB the base, the needle of the gal¬ 
vanometer will furnish by its deflec¬ 
tion the angle of the object, or parallax, 
ATB. A second angle ABT being- 
read at the other telescope, we obtain 
the distance from the equation :— 
AB sin ABT 
AT=- 
sin ATB. 
by means of tables arranged in func¬ 
tions of the two angles. 
The inventor subsequently contrived 
in the following manner to save him¬ 
self from the obligation of measuring 
the angle- ABT. Now when the object 
is at T', so that ABT' is a right angle, 
the sin ABT' is unity, and a knowledge 
of the parallax AT'B is sufficient for 
our purpose. 
: In the case of an object. situated 
obliquely at T let us call a the angle 
T'BT which the normal to the base 
makes with the line, of sight, and draw 
AC at right angles to BT. Then 
AC 
AT=- 
Fig. 3. 
AB cos. BAC 
sin, ATB 
sin. ATB 
