10 
corresponding galvanometers are at zero. When that occurs these two central 
needles are parallel to those of the telescopes to which they are attached. Pro¬ 
longations of the needles over the face of the chart indicate by their intersection 
the position of the object. 
The plan (fig. 8) gives a clear idea of the manipulation. The telescope L' is 
directed on the object T. The central post needle n is, by means of the Wheat¬ 
stone’s Bridge abcdfg , brought parallel to the telescope L. The intersection of n 
and m (the prolongation of L') at t is obviously the position required. 
Figure 9 gives the telescope L' and the chart in elevation. 
Fig. 9. 
Figure 10 shews the practical arrangement of the plotting scales n and m. To 
Fig. 10. 
