HORIZONTAL SECTIONS. 81 



from the map. In the other supposed case, the staff or 

 man should be placed at a station by the edge of the 

 water on the near side of the marsh, and in a similar 

 position when starting again from the other side ; the 

 space between the two points will of course be level and 

 the distance must be obtained as before. 



Sometimes also a great deal of useless labour may be 

 saved by going somewhat off the line, as round a wood 

 or a village, care being taken to note the horizontal 

 angles at which the line is left and rejoined, also those 

 and the distances between all the intermediate stations. 

 These being afterwards laid clown shew the line actually 

 levelled, and the distance between the points of de- 

 parture and return can be scaled off the ascertained 

 difference in height will require no alteration. 



Plotting from Angles. The method of plotting the- 

 odolite observations varies somewhat from that de- 

 scribed, and must be performed with great care for the 

 result to be satisfactory. A straight line is drawn on 

 the long strip of paper to represent the "datum line," 

 sea-level or otherwise, and another line vertical thereto, 

 near the left-hand end of the paper, which should be also 

 the most southernly end of the section. On the vertical 

 line, which corresponds to station 1, set off the height 

 above datum of the B. M., in the case supposed, 365 feet. 

 Then with a protractor of circular or semi-circular shape, 

 and made in horn or other transparent material, lay 

 down a line inclined to the datum line as many degrees 

 and minutes as are recorded in the first observation. 

 This being a " back-sight " and " depression " means of 

 course " elevation " in a forward direction, the line must 

 therefore rise from the bench-mark. The angle is best 



6 



