80 
MINTS To TIIAVELLeM 
rounds showing that the internal azimuth axis, about which it tui 4 HS i 
is truly vertical. Clamp the vernier-plate to the lower plate by turning 
the clamp-screw C, and loosen the clamp-screw E; move the instrument 
round its azimuthal axis, and if the bubble retains its central position 
during a complete revolution, the external azimuth is truly parallel with 
the internal; when this is not the case, the instrument must be sent to 
the maker, as this fault cannot be remedied by the traveller. 
It is most probable that the levels on the vernier-plate will now be 
found out of adjustment, and the bubbles must be brought to the middle 
of their run by turning the capstan-headed screws at the end of each 
of them. 
Horizontality of the Axis of the Telescope.— This is to be tested by the 
striding-level, which is supplied with the instrument. Apply it to the 
pivots y, and if the bubble is not in the middle of its run, bring it to 
that position by turning the capstan-headed screws t under the moveable 
bearing. If there is no striding-level, this adjustment can be tested by 
observing a long plumb-line, first making the intersection of the threads 
in the diaphragm coincide with this line, and then, if the point of 
intersection moves along the line when the telescope is elevated or 
depressed, the adjustment is perfect; if not, it must be made to do so 
by turning the capstan-headed screws. 
The adjustments can be tested in the following simple manner :—With 
the plummet supplied with the instrument, find the exact central spot 
over which the instrument stands; drive a peg into this place, and fasten 
a cord to the peg; now go in any direction, for say 40 feet, and drive in 
another peg, stretch the line tight between these pegs, and then intersect 
the line with the threads in the diaphragm, clamp the horizontal plates, 
and if the intersection remains perfect while the telescope is moved on its 
axis, the adjustments are so far correct. Next move the outer peg about 
90° (with the same radius) from its first position, and again drive it into 
the ground and draw the line tight as before; unclamp the vernier-plate, 
keeping the lower plate clamped, and repeat the previous operation; if 
the point of intersection of the threads in the diaphragm keeps on the line 
while the telescope is moved on its axis, the theodolite is in adjustment,, 
if not, the adjustments should be gone over again. 
The Vernier of the Vertical Limb .—When the foregoing adjustments have- 
been made, set the vernier of the vertical limb to 0° 0' 0", and bring tlm 
