24 
HINTS TO TRAVELLERS. 
graduated to read to 10". B is the Lower-plate; it is graduated into 
860°, each degree being again subdivided into 10', and can, with the 
vernier, be read to 10". These two plates combined are called the 
Horizontal limb , and revolve independently of one another, but when 
required, can be made to move together by tightening the Clamp-screw C ; 
the slow motion is obtained by the Tangent-screw D; the lower plate has 
also a Clamp E, and a Tangent-screw E. G G is the Tribrach System . 
H is the Horizontal axis . There are three Levelling screws , 1,1, I. K is 
the Tripod , on which the instrument is firmly screwed; underneath, in the 
centre, there is a hook (not shown in the drawing) from which to suspend 
a plummet in order to indicate the exact position w T here the station peg 
is to be driven into the ground. The vernier-plate carries a compass L 
in its centre between the supports of the Telescope 0 ; it is graduated into 
360°, and fitted with a screw M to lift the magnetic needle off its centre 
when not in use. The two Frames N N carry the bearings V for the 
telescope, with its level P, and the graduated circle R, called the Vertical 
circle, with its two verniers S S, and Microscopes m m. The vertical 
circle is graduated from 0° to 90° through one quadrant, then again 
from 90° to 0° in the next quadrant, and so on round the circle; the 
degrees are subdivided into 10', and, with the verniers, read to 10". 
Upon the other side of the vertical circle, in most instruments, are 
marked the number of links to be deducted from each chain, for various 
angles of inclination, in order to reduce the distances, as measured along 
the ground at these angles, to the corresponding horizontal distances. The 
horizontal axis of the telescope is formed of two cones, the larger ends of 
which are attached to the telescope tube, while the small ends, called the 
Pivots, p, are ground into two perfectly equal cylinders; the pivot which 
does not carry the vertical limb is pierced, and allows the light of a lamp 
to fall upon a small reflector (not shown in the drawing) which is screwed 
into the centre, on the axis of the telescope, and inclined to it at an angle 
of 45°, by which means the light is thrown directly down the telescope, 
and illuminates the fine threads, or web, attached to a Diaphragm inside 
the telescope, which is kept in its place and adjusted by the screws y y, 
of which there are four. The Lndex -bar, x, is fixed in its place by the 
Clip-screws, z z. The vertical-limb is furnished with a Clamp and a 
Tangent-screw, w; d d are Levels at right angles to one another; l and h 
are the small lantern and its holder, which fits into a slot in the frame 
