588 



MAGNETIC FIELD. 



1161. The theodolite being adjusted in the usual way, in the 

 geographical meridian, either by a polar star or by the corre- 

 sponding heights of a star or of the sun, etc., the sun is observed 

 by a blackened glass, and, instead of sighting one of the edges, it 



Fig. 240. 



is better to use a network, so that the image comes within four 

 rectangular tangent wires. 



Suppose, for instance, that we merely know the latitude of the 

 place. Let OP (Fig. 241) be the earth's axis, OA the vertical, OE 

 the direction of the star at the moment of observation, / the latitude 

 of the place or the height of the pole, h the height EQ of the star. 



