12 
Fig. 12. 
//! 
'/ ! 
/ 
// / 
/ /' / 
✓ /' / 
/ / / 
/ /’ / 
' / / 
/ 
These are all connected by the Wheatstone’s Bridge 12 3 4, and with the gal¬ 
vanometer Gr. Let CA and DB be the positions of the telescopes when pointed 
at the object T; let EA be the position of L when parallel to DB; and OAX 
PBY be normals to the base ; let the azimuthal angles of the object at A and B 
be XAT=a and YBT=/L When the telescopes follow the lines EA and.DB 
the galvanometer is at zero ; and if L be pointed at the object, following CA, the 
deviation of the galvanometer measures the arc EC, whence, as we have seen, the 
range can be deduced. 
Suppose, now, that we turn the arc ^180° round the axis OX, we shall have 
E at E', so that henceforward the galvanometer shall be at zero when L is at E'A. 
If then we point L at the object, following the line CA, the deviation of the gal¬ 
vanometer will be proportional to the arc CE', i.e., to the sum of the azimuthal 
0- + /2 
angles, a + f3. That sum can be read on the galvanometer, or-, the mean of 
2 
the two angles. 
Take E in the middle of the base, and we may say, with sufficient exactitude, 
a + /3 
that the azimuthal angle y at that point is equal to-The slight error invol- 
2 
ved consists in taking the line TF, which divides the triangle ATB into two 
equal parts, as coincident with the bisector of the angle ATB. As this angle 
never exceeds 3° the error involved is only 2'. If then a gun be situated at F 
