SURVEYING WITH THE THEODOLITE. 
119 
The third is a method of observing and recording the different 
directions of successive portions of a line, such as a boundary, or route, 
so as to read off on the instrument at each successive point or station 
the angle which the route or boundary makes with the first line observed, 
which is called the zero line, and not with the preceding line. 
The operation consists essentially of taking each back sight with the 
lower set of screws (which moves the theodolite without altering the 
reading) and taking the forward sights with the screws of the vernier, 
or upper plate , which moves the vernier over the arc measuring the new 
angle; and thus adds it to or subtracts it from the previous reading. 
Set up the theodolite at some station, as B (Fig. 2); set the vernier at 
360°, and by the lower set of screws sight back on A. Tighten the lower 
clamp, reverse the telescope, loosen the upper clamp, and sight to C by the 
upper set of screws, and then clamp the vernier plate again and record 
the reading. Remove the theodolite to C, sight back to B by the lower 
set of screws ( keeping the upper set clamped at the previous reading ), then 
clamp the lower motion, reverse the telescope, unclamp the vernier plate 
and sight to D by the upper set of screws, and record the reading. 
Then go to I) and proceed as at 0, and so on. The readings of the 
upper plate vernier give the angles measured to the right or “ with the 
sun,” as shown in the arcs in the figure. 
Care should be taken to keep the same side of the instrument ahead 
and read the same vernier throughout. It is advisable to take the 
compass bearing of each line of the route to serve as a check on the 
observations; for the difference between the magnetic bearings of any 
two lines of route should be the same approximately as the angles 
between them measured by the theodolite. The bearings also prevent 
any ambiguity as to whether the angles have been taken to the right 
or the left. 
Rounds of angles can be taken at each station for fixing the positions 
of objects along the route, which, like the line of route, must be measured 
from the first or zero line. 
Triangulating .—Although an explorer will seldom have time or oppor¬ 
tunity for carrying out the triangulation of any extent of country, there 
are occasions on which he may be able to do so, and though he cannot 
hope to make this class of survey with the detail with which govern¬ 
ment surveys are carried out, there is no reason, if he can spare the 
