12 
ttttfTS To TRAVELLERS. 
the prismatic compass, first adjust the prism by sliding it fib and 
down until the divisions on the circle are seen distinctly; if a tripod 
stand is used, screw the compass to the ball-and-socket joint, and 
move the instrument until it is perfectly horizontal (the same pre¬ 
caution must be taken if it is held in the hand); raise the sight-vane, 
until it is perpendicular; look through the slit in the prism-plate, 
and bring the thread of the sight-vane in a line with the object; 
wait until the magnetic needle comes to rest, and read the bearing 
through the eye-hole in the prism-plate. A bearing thus taken shows 
the angle which a straight line drawn from the observer, to the object, 
makes with the magnetic meridian, and is called the magnetic bearing. 
To get the true bearing the magnetic variation must be applied as 
follows :—If the variation is east add it to the bearing, if west subtract it, 
and the result in either case will be the true bearing. Thus : the magnetic 
bearing of an object was 160° and the variation 20° east, then 160° + 20° 
= 180°, the true bearing: the bearing of an object was 160° and the 
variation 20° west, then 160°—20° = 140°, the true bearing; but since 
the magnetic needle will be affected equally by variation within certain 
limits of time and space, the difference of the bearing of any two objects, 
taken from the same station, will be the angle subtended by them, as the 
difference in their azimuths will not be affected by the variation. 
Where possible, the bearings should be taken at both ends of a base, or 
line of bearing, the mean of which will be the correct bearing. When 
the sun’s azimuth or amplitude has to be taken, one of the dark 
glasses should be placed before the slit in the prism-plate, and the 
mirror should be moved on the sight-vane until the reflected image of 
the sun is seen in the mirror through the slit in the prism-plate; the 
bearing is then taken in the manner before described. Great care must 
be observed when using this instrument to avoid all magnetic rocks, as 
they may so affect it as to render bearings taken in their vicinity useless. 
Hypsometrical Apparatus . 
The boiling-point apparatus consists of a thermometer, A, generally 
graduated from 180°to 215°*; a spirit lamp, B, which fits into the bottom of 
* When they are intended to be used at very great elevations, the thermometers 
will have to be specially constructed with extended scales. 
