36-INCH INDIA THEODOLITE. 
25I 
between the tribrach and the plate being about T V inch. 
The object of this arrangement is to obviate the dis 
turbance of level and azimuth which arises from clamping 
foot screws of the ordinary construction after adjustment, 
as well as that due to looseness of the foot screws in the 
tribrach arms. The arms of the spring plate, being 
of considerable width, have great horizontal rigidity, but 
being comparatively thin are easily bent vertically. The 
outer ends of the arms rest on the cast-iron circle or 
stand ; the foot screws pass through the tribrach arms, 
but not through the spring arms. It is evident then 
that when the foot screw is turned inwards with the 
screwing motion, the solid end of the tribrach will be 
raised and the slit between the two arms widened; 
but since the end of the screw does not rest on the 
stand, but on an intermediate arm, which is actually a 
portion of the tribrach itself, it is clear that if a lateral 
pressure be applied to the tribrach, no motion will be 
caused thereby, however loose the screw may be, so 
long as the pressure is less than the lateral rigidity of 
the intermediate arm. The lateral pressure generated by 
turning the instrument in azimuth when taking observ 
ations is greatly within this limit. This plan is perfectly 
successful, but it is only available where a moderate 
range of vertical movement is needed. In the present 
instance, as the cast-iron ring or stand on which the 
instrument is supported is always first made sensibly 
level, the vertical range of the foot screws need not be 
more than a small fraction of an inch. Another point 
with regard to the foot screws is their delicacy and 
certainty of action. This is attained by applying to 
them a clamp and tangent screw arrangement H very 
similar in principle to that sometimes applied to circles. 
Although the foot screws themselves are rather coarse, 
having only about eight threads to the inch, the arrange 
ment is such that one entire revolution of the slow