14 
object. If also the battery is not exactly on the base, the galvanometer will shew 
a slight constant error, which can be rectified beforehand by special adjustment. 
V. Electric Apparatus for Indirect Laying. 
In figure 14, let P be the gun ; T the object, visible only from the observatories 
A and B ; and DD the chart representing these four points, and itself situated 
either at A or B, or elsewhere. The arcs H and H ; have the same resistance as 
h and li' on the chart. The scales m and n are to be made respectively parallel to 
the telescopic needles at A and B. For that purpose the arcs H and li are con¬ 
nected to an electric battery, and to the galvanometer g , by the Wheatstone’s 
Bridge, 1 2 3 4, in such a manner as to bring the galvanometer to zero by means 
of the parallelism of m, and the needle and telescope at A. A reciprocal arrange¬ 
ment connects H' and h'. 
When then both the galvanometers are at zero, and the telescopes are on the 
object, the intersection of m and n gives the position of the object on the chart. 
Fig. 14. 
/ \ V; 
/ 1 \ 
/ \ \ 
