FIELD GEOLOGY. 



and 8 others sub-dividing the spaces between the car- 

 dinal and intermediate points, as KKE., and so on. To 

 determine a locality, the needle must be set free, and 

 the instrument held perfectly level in front of the 

 observer, and between him and the distant object on 

 which he takes a bearing. It must then be turned 

 steadily round, until the needle comes to rest 22 to 

 the left or West of North. At the same instant tho 

 eye, being carried from the centre of the compass to the 

 object and back again, will detect the point in the cir- 

 cumference on a line from the object to the observer. 

 This reads off perhaps N.E., or half-way between N". and 

 E. ; this would be represented on the map by a line, in 

 such a position as to lie mid-way between two others, 

 one vertical for N'. and S., the other horizontal for E. and 

 W. direction. A scale or pencil laid across the object 

 on the map in such mid-way position affords a means 

 of drawing a pencil line that corresponds with the bear- 

 ing taken. The observer is situated at some point along 

 this line, and by repetition of the observation on another 

 object (as nearly as may be at right angles to the first) 

 he gets a second line crossing it at a point which repre- 

 sents the required position. If E.N.E. had been read off, 

 half-way again between N.E. and due E. would give the 

 direction, and the same method of course applies to all 

 the other points in the compass. 



The prismatic varies from the pocket compass in 

 having its circumference divided into 360 degrees, 

 instead of into cardinal and intermediate points ; and 

 in being provided with sights for taking more accurate 

 observations. The needle carries with it a nicely 

 balanced card on which the divisions are marked, the 



