SURVEYING WITH THE THEODOLITE. 
121 
the side C D through each triangle, thus verifying the result; then 
choose two points, E and F, the prolongation of A B, so that the triangles 
c 
C D E and CDF may be well conditioned. Observe all the angles in 
these two triangles, and calculate E F twice through the separate 
triangles. 
When the prolongation can only be conveniently effected in one 
direction, as towards F, a corresponding method can be adopted, which 
differs only in being one-sided. Choosing points C and D, rather more 
towards F, and observing all the angles, compute B C and B D; then, 
choosing F, so that CDF may be well conditioned, and observing all 
the angles, compute B F both in the triangle B C F and in B D F, thus 
verifying the result 
Having selected and measured a base, set the theodolite up immedi¬ 
ately over one end of it, and see that the ends of the tripod legs are well 
thrust into the ground, or better still, placed on pegs driven well into 
the ground. Level the instrument carefully, and get rid of parallax in 
the manner described, p. 26. Set the vernier of the vernier plate 
accurately to 360°, and then unclamp the loiver plate, and keeping the 
vernier clamped at 360°, move the telescope round until the intersection 
of the threads of the diaphragm are nearly on the mark at the other end 
of the base. Clamp the lower plate, and by means of the lower tangent 
screw, cover the mark with the intersection of the threads in the 
diaphragm; now release the clamp of the vernier plate and turn the 
telescope on each point in succession which it is desired to fix, moving 
