ADJUSTMENTS OP THE THEODOLITE. 
35 
underneath the vernier-plate by means of the hooks COO provided for 
that purpose. Set the vernier of the horizontal plates to 360°, and 
then keep the upper plate clamped. Unclamp the lower plate and 
turn the whole instrument round until the magnetic needle points 
nearly to the central division in the box, clamp the lower plate, 
and make the needle point exactly to this division. The telescope will 
now point to magnetic North, and if the upper plate is unclamped 
and turned on to any object, its magnetic bearing can be read from the 
verniers. Care must, of course, be taken to keep the lower plate firmly 
clamped. 
P is the striding level which can be used in levelling the transit axis. 
G is the lantern which is placed on the stand H after it has been fixed to 
the standards, and is used to illuminate the threads of the diaphragm 3 
through the hollow axis K 3 when star observations are being taken. 
Tacheometer . 
A Tacheometer is an instrument for measuring small angles. Of 
the many different types of tacheometers in use by surveyors the form 
adopted by the Indian Government, and made by Messrs. Troughton & 
Simms, is best suited to meet the requirements of the traveller. It con¬ 
sists of a telescope A, fitted with a pair of micrometers , B B, which are used 
for measuring either vertical or horizontal angles, as they can be turned 
through an angle of 90°, and fixed in that position by the screw C. 
The telescope is mounted on standards D D, over a prismatic compass E, 
and is furnished with a small circle, F, for taking vertical angles, which 
can be read to minutes. G is the screw by which it is clamped in alti¬ 
tude ; H is the vertical slow motion screw. The instrument is fitted with 
a screw (not shown in the plate) for clamping it horizontally, and I is the 
horizontal slow motion screw. The bearing of any object is read through 
the prism N. There are three levelling screws , K, which fit into a 
tribrach L, that screws on a tripod M. The instrument is levelled 
by means of the screws K, and a level attached to one of the standards 
(not shown in the plate). 
There is a disc of glass visible in the field of view, divided in such a 
d 2 
