48 ON MAGNETISM. 



diverging from the wire or mark in C and falling on 

 the object-glass in D will emerge from it in a parallel 

 pencil, every ray being parallel to that which passes 

 from the mark G through the center of the object-glass 

 in D: or parallel to the optical axis of the Reversed 

 Telescope CD. A theodolite E must be placed at 

 any convenient distance so that the object-glass of its 

 telescope shall receive the whole or a part of the pencil 

 of parallel rays coming from D : the eye, applied to the 

 theodolite-telescope, will see the mark C distinctly : the 

 theodolite may be slightly turned till the wire in its 

 field of view is seen to coincide with the image of the 

 mark C: and then the graduations of the horizontal 

 circle of the theodolite indicated by the pointer or 

 vernier of the rotatory part of the theodolite (which 

 carries its telescope) are to be read. After this, the 

 theodolite is to be turned with its telescope elevated 

 at the proper angle, as at E', to view the pole-star or 

 other circumpolar stars; a meridional direction being 

 thus obtained, the horizontal circle of the theodolite is 

 to be read as before: and the difference between this 

 reading and that formerly taken when observing the 

 mark C is the magnetic declination. 



But there are some delicate points requiring atten- 

 tion, which it may be well to describe here. First: the 

 line along which our theodolite-telescope is directed is 

 the optical axis of the Reversed Telescope CD. But 

 the line which takes the direction of Terrestrial Mag- 

 netism is an undiscoverable line in the interior of the 

 magnet, called its "magnetic axis." How can we be 



