IN THE MOUNTINGS OF THE TELESCOPES AT BIRR CASTLE. 
157 
nut, and lock-nut on the uppermost trunnion. It will be observed that the sides of 
the tube are expanded to 50 inches at the trunnions to give greater steadiness. The 
diameter of the speculum is about 36 inches, but the mouth of the tube has a clear 
aperture of 39g inches, so that no light may be cut off within a field of 40' diameter. 
It will be observed that, unlike Mr. Lassell’s and that of the great Melbourne tele¬ 
scope, the tube is square. It also differs from Mr. Lassell’s in this—that the brass ring 
(G, Plates 12 and 13) at its upper end is the only part which can be turned round the 
axis of collimation, but this motion meets all requirements of convenience in observing. 
The ring is retained in a nearly central position by three blocks of brass, provided with 
shoulders, against which its inner edge rests. The outer edge of the ring is chamfered 
off, and each block is provided with a screw (S) and a piece of brass, by simply 
tightening or slackening any one of which the ring is made free to move or firmly 
clamped. 
The tube is made of angle iron, the longitudinal bars 1-g-X 11X ^--inch scantling, and 
the lattice bars l|-X l-g-X-^-inch above the trunnions and 1 § X lb X ^-inch below 
them. The upper end of the tube for a length of 12 inches is made of plating of 
about Y^-inch thick, and cylindrical in order that the corners may not interfere with 
the apparatus attached to the eyepiece. The supports at the back of the speculum 
continue the same as they have been since it was first mounted, and it has on that 
account been considered unnecessary to complicate the drawings by representing 
them.* The pressure on its edge in every position is now borne by a ring of angle 
iron, suspended from the speculum box by eight rods, linked to it as shown in dotted 
lines (lm, l m) in Plate 11, fig. 4, those which happen to be uppermost taking the weight 
of the speculum, and the ring is free to move parallel to the axis of the tube as much 
as may be rendered necessary by the elasticity of the back supports. The eight rods 
pass freely through holes in the framing of the tube and are screwed and fitted with 
nuts to tighten them, so that the position of the speculum may change as little as 
possible on passing from one side to the other of the zenith.t The speculum box is oi 
cast-iron, and it has a second bottom of wood, for the purpose of enclosing an air-space 
to be heated by a very small lamp, so that a small amount of heat may penetrate 
through the first or cast-iron bottom to the speculum and thereby diminish the chance 
of deposit of dew upon its surface. The space between the upper edge of the box and 
the edge of the speculum is closed in with wood and canvas. 
The coarse motions both in Eight Ascension and in declination are effected by 
means of worm-wheels of a pitch of two threads to one inch, bolted respectively to the 
lower end of the fork and to the extremity of one of its branches. The worm geering 
into the latter is fixed on to an arm turning on the trunnion and held in position by a 
screw of eight threads to one inch pitch, which gives the fine movement in declination 
* Their principle has been fully described in the Phil. Trans, for 1861, Part 3. 
f A collimator has been fitted to the interior of the tube to enable the position of the speculum to be 
corrected, 
