MEASURE OF 'MAGNETIC DEFLEXIONS. 



59 



while B takes its direction under the action of terres- 

 trial magnetism only, the mark C is viewed with the 

 telescope H and its image is made to coincide with the 

 wire in H\ in this state, the graduations of Sunder the 



pointers/,/, are read. A is then mounted in its place, 

 with its red end to the east, suppose, as in Figure 24 : 

 the north end of B immediately turns to the west : the 

 bar F is turned carefully in the same direction, till the 

 mark G is again seen by the telescope H, and its 

 image coincides with the wire in H: then the gradua- 

 tions of E under / /, are read. The difference between 

 this reading of the graduations of E and the former 

 reading is the angle through which B has been deflect- 



