74 FIELD GEOLOGY. 



Plotting from heights. Before considering a dif- 

 ferent style of taking surface levels, it will be well to 

 see how the results of the proceeding and similar obser- 

 vations are plotted ; that is, reduced to the form of a 

 representative drawing. The method in all of them is 

 the same, a straight line is drawn on the long slip of 

 paper usually employed, to represent the datum-level, 

 whether it be the sea-level, 1000 feet below it, or any 

 other that may be adopted. Along this line are set off 

 by scale the distances of each station in succession 

 and at right angles to it lines in pencil are drawn 

 through all these points. The reduced level of each 

 station is then scaled off along its vertical line and 

 another line is sketched in, running through all these 

 reduced level points, and as nearly as may be conform- 

 ing to the shape of the ground between. This last 

 operation will be facilitated and greater approach to 

 accuracy secured by sketch lines made in the field on 

 the plain side of the level-book. 



If the section is being made from ordnance bench- 

 marks, the proceeding is exactly the same if from 

 contour maps, with this difference only, that every point 

 where the line crosses a contour is considered a station, 

 the distance scaled off and the height inserted accord- 

 ingly. 



By Theodolite. The method of taking levels by the 

 theodolite differs from that by the level, in the former 

 having its visual line not in a horizontal plane, but 

 inclined at various- angles to the horizon according to 

 the slope of the ground between the stations. The 

 telescope, cross- wire, and spirit-level are the same, and 

 the instrument may be used as a level if desired. But 



