80 FIELD GEOLOGY. 



then be ascertained by means of two (or more) observations 

 with the theodolite ; one at some distance from the 

 base, and the other nearer thereto by a few chains the 

 length and the fall of the ground between the points 

 must of course be accurately determined. One of the 

 observations can be made from the last station, and the 

 other in advance of it, the sights being taken on the staff 

 or other object upon the summit. These with the length 

 and fall between the points being plotted as a diagram, 

 and with reference to a temporary datum line, will 

 give at the intersection of the angles the height and 

 position of the object relatively to such datum (see fig. 18). 



Fig. 13. 



Various expedients may with advantage be occasion- 

 ally adopted tinder exceptional circumstances, as for 

 instance, in coming to a field of standing corn, which 

 would be injured by the chain-men measuring through 

 it, or a tract of marsh land flooded so as to be impassable. 

 In the former case the exact level of an accessible object 

 on or near the line may be transferred across the field, 

 by clamping the instrument at zero, and noting where 

 the cross-wire cuts some object on the other side ; the 

 distance must be judged, or it may perhaps be scaled 



