COMPASSES. 
11 
stud S, by pressing which with the finger the ring is brought to rest; 
it also serves to check the vibration of the needle. The box E has a 
cover I, which fits either the top or bottom, in which latter position it 
is shown in the drawing, and with it the instrument can be held when 
taking an observation. The prismatic compass is frequently fitted to 
screw on to a light tripod, with a ball and socket adjustment, and can 
then be used with greater accuracy either for taking bearings, or as an 
angular measuring instrument. 
Pocket Compass . 
A prismatic compass is not suited for taking bearings, except 
through the prism, on account of the reversal of the figures, and their 
arrangement from the south point; it will therefore be convenient, for 
taking rough bearings, for the traveller to provide himself with a pocket 
compass, having a card of the size and pattern, shown above; it should 
be made of aluminium, which is both light and strong. The compass box 
should be fitted with a lever to throw the magnetic needle off its centre 
when the compass is not in use, and the glass should be thick, flat 
crystal. For night work a luminous pocket compass will be found useful. 
Observations with the Prismatic Compass To take an observation with 
