122 
HINTS TO TRAVELLERS. 
the telescope from left to right, and recording the angles in the field 
book. Having completed the first round of angles, move the instrument 
to the other end of the base, and the end at which the first round of 
angles was taken will now have to be made zero, and another round of 
angles taken in the manner just described. The reading off the angles 
should be taken on the vernier originally set to zero, or the readings of 
both verniers, and, if they differ by more or less than 180°, taking the 
mean as the correct reading. 
In fixing points in the above manner, care should be taken, where 
possible, to select two points which will serve for a base in carrying on 
the triangulation, and the angles of elevation should be taken, face right 
and face left, to all peaks or points the heights of which it is wished to 
determine. After each round of angles, the telescope should be directed 
on zero, and the vernier of the vernier plate should then read 860°; if it 
does not, the instrument must have been moved, and the round of angles 
must be taken again. Accuracy will be insured by repeating the 
measurements of the horizontal angles. This is done by moving the 
vernier forward, say 1° with the upper set of screws, and again directing 
the telescope on the zero point with the lower set of screws, then taking 
the round of angles again, which, if correctly taken, will differ from 
those of the previous round by exactly 1°. It must be remembered that 
the upper screws are used for setting the reading to 860°, and that the 
zero point is always made with the lower set of screws, which latter 
must not be touched again until after a round of angles has been taken. 
The bearing of the base line must be taken, and the best way of doing 
this is by determining its true bearing from its angular distance from 
the sun, as shown pp. 206, 207, roughly by taking its bearing with the 
magnetic needle. 
In using a theodolite in exploring, it has generally been found very 
advantageous, when taking rounds of angles, to set up the instrument 
so that all recorded readings are magnetic bearings. This is done in 
the following manner: Having levelled the instrument, set one of the 
verniers of the vernier plate to 360°, and clamp it, release the clamp of 
the lower horizontal plate and move the whole instrument round until 
the north end of the magnetic needle steadily points to the north in the 
compass-box, or trough, and then clamp the lower plate, release the 
vernier plate, and all readings will now be magnetic bearings. There 
