LEVELING 
mined with some care with a view to its general use in this survey, 
it is called a bench-mark. 
The instrument is now directed toward the second point, X, 
care being taken to make sure that the instrument tube remains 
in exactly the same horizontal 
plane. The leveling-rod is now 
set up with its zero mark on 
the point X and the rod is read 
again. This reading shows the 
height of the telescope tube 
above X; but the desired in- 
formation is secured by sub- 
tracting the second reading from 
the first, the difference thus 
obtained being the difference of 
elevation between A and X. 
Methods of Leveling 
Differential leveling con- 
sists in finding the difference 
of elevation between two or 
more points in the manner 
already explained. 
Profile leveling consists in 
taking the elevations of a series 
of points along a given line. 
As a matter of convenience | Fig. 19. Ustna THE Hanp LEVEL 
only these readings are made 
at regular intervals, as 10, 50 or 100 feet. The information thus 
secured is best exhibited when plotted out as a profile. One may 
thus construct a profile which shows the fall of a stream or the 
grades, up and down, of a road. See Figure 20. 
Cross-section leveling consists in running several profile levels 
47 
