130 MEASUREMENT OF BASE LINE, 
standard bar, and its length accurately determined. The result 
of the several comparisons, and the temperatures at which they 
are taken are given in the Appendix A. It will be seen that 
two tapes (A and B) were used in the measurement of the base, 
and the length adopted for each tape was the mean of twelve 
comparisons taken at different times. The apparatus referred to 
is similar to those used for comparing bars of assumed equal 
lengths. It has micrometer microscopes attached by which the 
smallest differences between the bar and each tenth part of the 
tape, with which the comparison is made, can be determined. 
The temperature is observed by means of the best thermometers, 
the index errors of which are known. 
The base line is situated on the open plain known as the 
prairie plain, between Mount Irving and Mount Maria, the 
summits of which are the terminal points. The former mount 
is about 215°8 feet, and the latter 162°5 feet above the level of 
the plain. The principal reason for selecting Mounts Irving 
and Maria, as terminal points of the base, were that observations 
could be made more accurately from their summits than from 
the terminals on the plain, where the atmospheric conditions 
were less favourable for the use of telescopes of high mag- 
nifying power. At first it was decided that the base should be con- _ 
fined to the plain, on the line extending from one summit to the 
other—the length of the sections at each end of the base being 
computed by triangulation. When the measurement commenced 
it was proved to my satisfaction that the terminal sections could 
be measured without risk of error. The ascent to the summit 
of Mount Maria is a gradual slope, so that no difficulty was 
experienced there. Mount Irving is rocky and more abrupt, 
necessitating a track being cleared and levelled In the case 
of the latter section the measurement was carried out three 
times—two of them in opposite directions. The length was also 
computed by means of triangulation, as a check upon the work. 
The plain was, moreover, very suitable for the operation in so 
