1 37 
of a Transit Circle . 
steady, by the levels on the bottom plate never altering in the 
least upon screwing the clamps. 
The four pillars, and their braces, explain themselves. 
They stand over the bell-metal circle ; and the clamps are 
placed near the foot of each, for greater steadiness ; since 
they carry the Y s for the pivots of the transit. 
The construction of these Ys is peculiar : they hang, as it were, 
in gimmals, though of a very firm kind, a drawing of which 
(Tab. XVI. fig. 1) will best explain them. They have a hori- 
zontal motion, smooth and steady : the T, or frame, AB, which 
carries them, turning on a perpendicular axis of 2^ inches, CD, 
ground to its socket; on the outside of the plate EF, which 
connects them with the pillars, and resting on that plate, to 
which the bottom of the frame itself is ground likewise. In 
this frame they have a vertical motion : the Ys themselves 
carrying a horizontal axis at AB, which, consisting of two 
frusta of cones on each side, in contrary directions, with a 
collar over them, guards against any shake whatsoever, while 
it admits of the Y adapting itself to the direction of the pivot. 
The idea of hanging them in this way, as well as that of 
turning the whole instrument in azimuth on a ground plate, 
was suggested by our late member Mr. John Smeaton ; to 
whom the world has, during many years, been indebted for re- 
peated capital improvements in mechanics. 
By thus hanging the Ys, the pivots have a bearing on them 
from end to end ; instead of riding on a bell-metal ridge, as 
is the usual method where the. Ys are fixed, and cannot set 
themselves in the direction of the axis. This seems to be a 
better bearing, and much less likely to wear the pivots. 
Yet, to guard against any wear, a pair of cylindrical 
