120 
HINTS TO TRAVELLERS. 
time, why he should not attain a considerable amount of accuracy and 
do good preliminary work. 
The first point to which he must give his attention is the selection 
of his base, which must bear a fair proportion in length to the distance 
of the points he desires to fix, and must be so situated with regard to 
those points as to give well-conditioned angles. 
If the country in which he finds himself is open, and fairly level, the 
difficulty of measuring his base, with the chain or steel tape, will not be 
very great, but care should be taken, as the accuracy of his survey will 
depend on the length of his base being correctly known. If the ground 
on which the base is measured is sloping, the distance measured must 
be reduced to the horizontal in the following mannerObserve the 
angle of slope with the theodolite and read, on the back of the vertical 
circle where they are usually given, the number of links which have to 
be subtracted from each chain to give the horizontal measurement of 
the base. If these figures are not given at the back of the vertical 
circle, then the horizontal distance must be calculated, with the observed 
angle and the measured distance as the hypothenuse of a right-angled 
triangle. In some mountainous countries it is quite impossible to 
measure a base in the usual manner, in which case the Bar-Subtense 
system, described pp. 37 to 40 and 111 to 116, may be used with advan¬ 
tage. There are places where the country is so densely wooded and hilly 
that it is next to impossible to get a measured base, in which case resort 
must be had to a geographical base as described by Sir Charles Wilson, 
p. 90; but as the length of such a base depends entirely on astronomical 
observations, which will in all probability, under the circumstances, con¬ 
tain errors, it is not a system to be recommended if it can possibly be 
avoided. It may frequently happen that considerable difficulty would 
be experienced, owing to the nature of the ground, in measuring a base 
of sufficient length to give well- conditioned angles from each of its ends 
to the points to be fixed, but if only a portion of the base is measured 
it can be extended by calculation without measurement, by either of the 
following methods:— 
When the measured base A B can be conveniently prolonged in both 
directions towards E and E, select two temporary stations, points C 
and D, so that the resulting triangles A C B and ABB may be well 
conditioned; observe all the angles of these two triangles and calculate 
