13 
the galvanometer, which in the first position of the arc h gave the range, now 
gives the direction. 
The following ingenious artifice produces these results without turning the arc 
h round the axis OX (Fig. 13.) In point of fact it suffices to reverse the wires of 
Fig. 13. 
the Wheatstone’s Bridge, with the result that the galvanometer then reads the 
sum, and not the difference, of the azimuthal angles. To effectuate that, the two 
arcs are connected by two Bridges, of which the first, 1 2 3 4, is installed in the 
ordinary manner, i.e. } so as to establish communication between the homologous ex¬ 
tremities of the arcs, and with its galvanometer G, placed at the gun, and graduated 
for distance. The second Bridge, 5 6 7 8, connects the left extremity of the arc h 
to the right extremity of h', and reciprocally ; its galvanometer G', also at the 
a-p/3 
gun, measures the sum a + /?, and is graduated in values of-. That is to 
2 
say, that in this case we read at the gun the direction, and with the other Bridge 
the range. 
The middle point of the base has only been selected for convenience of demon¬ 
stration. It would be easy to determine between what limits the battery could 
be suitably placed, and in particular, we should improve the accuracy of the direc¬ 
tion in bringing the battery to the point where the bisector of the apex angle 
meets the base, that is, towards the observatory B, which is the nearest to the 
