68 FIELD GEOLOGY. 



taken at each " reduced," by addition or subtraction, to 

 its height above or below the B. M. the height of this 

 being known, the levels are at once reduced to " datum- 

 level." 



The level consists of a telescope supported on three 

 legs, and having a fine cross-wire placed horizontally in 

 the line of sight. A spirit-level is fixed on it parallel 

 to its axis, by means of which, and three (or four) plate 

 screws, the telescope can be adjusted so as to be perfectly 

 horizontal in whatever direction it may be turned. The 

 line of sight thus represents a horizontal plane, and the 

 cross-wire must necessarily, within certain limits, be 

 seen to intersect a distant object in the same plane with 

 itself. The observations are taken by means of a staff 

 placed on the stations in turn this staff is capable of 

 being extended to 14 feet in height and has one side 

 divided into feet and hundredths. Through the telescope 

 with the ordinary eye-piece it is seen inverted, but the 

 ye soon gets accustomed to reading it in this position ; 

 and being magnified the point cut by the cross-wire can 

 be accurately read off. 



The distances apart of the various stations can be 

 measured by chain, or scaled from the map on which 

 the line of section has been laid down the correctness 

 of the height at each station is not affected by want of 

 accuracy in this respect, as it is when taken by the 

 theodolite. The instrument should always be placed 

 about mid-way between the two places at which back- 

 and fore-sights are being taken, to minimise the slight 

 error arising from " curvature " and " refraction." 



As an example of levelling by this instrument we 

 may take the line of section running across the area 



