THE GEEAT MELBOTJENE TELESCOPE. 
151 
fulcra of these levers are attached to the sides of the cube of the polar axis ; one end of 
each bears up the larger frame (Plate VIII. fig. 29), as at a, a'; the other is attached 
to the end of a cross lever dropped on the stud h (Plate VIII. fig. 30), and fastened by a 
nut which is tightened till the elasticity of the levers acts with sufficient power to give 
the required relief of the friction. This is seen in Plate XII. fig. 15.] 
As the size of the rollers is limited by the dimensions of the cube C (Plate XII. fig. 15), 
they are not quite as effectual as the antifriction apparatus of the polar axis ; for it re- 
quires 12^ lbs. acting at the leverage of 20 feet to turn the telescope round this axis. 
Still one man can raise the telescope from the horizon to the zenith in twenty seconds. 
In reversing it from the east of the pier to west, or vice versa, two men are necessary for 
quick work, as it must be moved in P.D. as well as At ; they do it in forty-five seconds. 
The tangent movement in declination is so smooth that a star can be nicely bisected 
by it. The polar-distance circle P (Plate XII. fig 15), divided like the others on the 
alloy of palladium and silver, is 30 inches diameter, and has two verniers which read 
to 10". 
[The clock is lodged very compactly in a hollow in the pier ; a shaft passes from it gear- 
ing with a vertical one which drives the screw of the sector. It is regulated by a conical 
governor, shown in Plate VIII. fig. 31 (scale 3 inches to 1 foot), a is a double fork-shaped 
frame of gun-metal, in which hang the two T -shaped pieces V, V carrying the balls A, 
A', 5 inches diameter, of hollow brass filled with lead. These, when not in action, are 
retained near their working angle 45° by the piece c. Attached to the steel bars are 
small brackets, d , d 1 , carrying screws with divided heads, e, e', the extremities of which 
terminate in small cups to contain pieces of hard leather. These, when the balls attain- 
ing their speed fly out, rub on the disk B, the plane of which is set at right angles to 
the governor’s axis of rotation, and by their friction prevent any increase of speed. The 
lower bearing of the governor’s spindle is attached to a sliding-piece C : this slide is 
worked by a cam, f, actuated by the wheel and sector, g. The spindle of this sector 
projects in front of the clock-frame, where it carries an arm playing round a gradu- 
ated arc, and kept in any required position by a pin and a circle of holes. The initial 
adjustment of this clock is made by the nuts h, h 1 , which alter the working length of 
the conical pendulum, and the screws e, e', which alter its working angle. When 
the clock is adjusted to sidereal time, these require no alteration; for small differences, 
as in the case of planets, are corrected by the cam apparatus without stopping the 
clock. For the moon, the change to mean lunar time is made by a set of differential 
wheels, which are brought into play by the simple movement of a lever in an instant, 
and the final adjustment for the lunar rate at the time of observation is made by 
the cam.] 
Mr. Grubb had intended to supplement the action of this governor by an elegant 
contrivance controlled by an ordinary seconds’ pendulum, which was to add or subtract 
motive power as the clock lost or gained, but on trial we did not consider it neces- 
sary. The clock has a great excess of power over any probable resistance that may 
mdccclxix. x 
