16 
HINTS TO TRAVELLERS. 
140°; each of these degrees, according to the radius of the instrument, 
is divided into 10' or 20', and these are subdivided by the vernier D 
into 10" or 20"; these divisions on the arc are continued a short dis¬ 
tance on the other side of zero (0°) towards E, forming what is termed 
the arc of excess. The index is secured to the arc by a clamp screw G, 
which must be released when the index has to be moved over a large 
portion of the arc. In order to obtain the slow motion necessary for 
the accurate measurement of an angle, a tangent screw , H, is fixed to 
the index, but does not act until the index is fastened by the clamp screw. 
I is a fixed plane glass, the lower half of which, next to the frame 
of the instrument, is silvered, and the upper half left clear. It is called 
