THE SEXTANT. 
15 
the readings taken simultaneously at both stations; but it frequently 
happens that this is impossible, in which case the observations should 
be taken in the following manner:—State date and hour of observation; 
take the reading of the aneroid and the temperature of the air, in the 
shade, at the lower station; repeat this at the upper station, and again 
at the lower station on returning to it, but before taking this last reading 
a short time should be allowed to let the aneroid take up its proper 
working, as a descent will always, in a greater or less degree, affect it, 
unless a Watkin aneroid is used, which is said to be free from this 
drawback. 
In observing with the aneroid, the instrument should always be in the 
same position, as, for instance, with its face vertical; merely altering the 
position affects most aneroids with a very sensible difference of reading. 
On leaving a station to which it is not intended to return, the reading 
of the aneroid should be taken, and the temperature in the shade; during 
the day’s journey the difference between any reading and that taken 
at starting will approximately give the difference of height unless there 
has been some atmospheric change. This is only a very rough way of 
ascertaining whether a party, passing through a hilly country, has ascended 
or descended; for the accurate method of computing the difference of 
height of two stations, see examples (pp. 215, 216). 
The Sextant. 
The principle on which the sextant is constructed is this:—that the 
angle between the first and last directions of a ray which has suffered two 
reflections in one plane, is equal to twice the inclination of the reflecting 
surfaces to each other. The arc on which the angle is measured must 
therefore be divided into double the number of degrees which properly 
belong to an arc of the same extent. With this instrument we can 
measure the angle between two objects, in whatever direction they may 
be placed, provided the angle is within its limits. 
With the aid of the following figure, the different parts of the sextant, 
with their names, may be distinguished. 
A is a plane mirror called the index glass ; it is set in a frame, and is 
fixed on a centre perpendicular to the plane of the instrument; it moves 
with the index bar B C, the end of which, C, slides over the arc E F, which 
is graduated (on an inlaid plate of platinum or silver) from 0° to about 
