GEOLOGICAL MAPPING. 9' 



figures thereon being reversed so that the prism (which 

 inverts the rays passing through it) presents them to 

 the eye in their proper position. The card is, or ought 

 to be, so attached to the needle that proper allowance 

 has been made for magnetic variation. To take a bear- 

 ing, the needle must be liberated, the vertical sight 

 erected, and the prism pulled up to suit the eye of the 

 observer. The instrument is then held up to the eye, 

 being kept as level as possible in the hand, and directed 

 to the object from which the bearing is being taken, 

 until the card shall have gradually ceased to revolve. 



I I M I I I I II I I I I I I I Ijfc I I I I .1 I I I I I I I I I I I LJ 



Fig. 2. Protractor, upper side (half size). 



The division then seen immediately beneath the vertical 

 wire records the number of degrees subtending the 

 angle contained between the line of bearing and the 

 true N". and S. meridian. The number of degrees count 

 to the right of due N. ; thus E. reads 90, S. 180, W. 

 270, and 1ST. itself 360, the complete circle. In plotting 

 the line of bearing therefore the protractor (fig. 2) must 

 be laid on the map to the right of the object, the centre 

 of the semi-circle which it represents resting directly 

 thereupon and its inner edge parallel (as near as the 

 eye can judge) with the margin of the map that is, 

 due N". and S. A point is now marked on the paper 



