10 
HINTS TO TRAVELLERS. 
2. Instruments, and their Adjustments. 
Compasses. 
Prismatic Compass :—This instrument consists of a magnetic needle, 
A, balanced on a pivot, B, carrying an aluminium ring, C, divided 
into 360°; it is graduated from the south pole of the needle,—by 
west, north, and east to south again, from 0° to 360°; the 0° is not 
shown on the ring, since it coincides with 360°. A prism, D, is fixed 
on one side of the box, E, mounted on a hinge-joint, d; it can be 
turned down when not in use, and is attached to a plate, e, which 
slides up and down to suit the vision of the observer. In the plate 
Prismatic Compass. 
there is a slit through which the observer looks; it has also an arm 
with two dark glasses E, to protect the eye when taking a bearing 
of the sun. On the opposite side of the box is a sight-vane G, having 
a fine thread down its centre, and a mirror H, which slides on and off as 
required; it can be used with its face up or down, so as to reflect images 
of objects which cannot be directly observed. The sight-vane is also 
fitted with a hinge-joint, and when shut down, presses on a lever, which 
lifts the needle off the pivot. In front of the sight-vane there is a small 
