58 



ON MAGNETISM. 



carrying the needle B. To B are attached a frame C 

 with cross- wires and a frame D with object-glass, as in 



Fig. 24. 

 H 



Article 23, forming a Reversed Telescope, .fi" is a tele- 

 scope firmly fixed to F, at the same height as CD, which 

 can therefore see distinctly the mark G. A is the 

 magnet for which the proportion of magnet-power to the 

 Earth's horizontal magnet-power is to be found : it may 

 sometimes be placed in the position A': it is so sup- 

 ported that the view of the telescope H is not interrupt- 

 ed. In the Figure, the telescope is supposed to point 

 to the north, but it may equally well point to the south. 

 In the case now under consideration A is presented 

 broadside-on to B. The observation begins without A, 

 as in Figure 23 ; F is turned to such a position that, 



