THE GREAT MELBOURNE TELESCOPE. 
149 
acting upon this face is about five tons, any portion, or all of this, is relieved by the 
lever-apparatus on polar pier as seen in general drawing (Plate XII. fig. 15). The sector 
for relieving the lateral pressure of this bearing with its slide-box, springs, and adjusting- 
screw is seen at A. This sector is 27 inches radius, and works on a 1-inch hardened 
steel pin held in a cast-iron frame sliding in an outer box. This frame is forced up by 
the screw d, acting through the lamina of steel springs e. This sector relieves any 
desired position of the six-tons pressure in this direction, the friction due to this being 
reduced in the proportion of 54 to 1. At H are seen the fixed and differential hour- 
circles, 34 inches diameter, the lower (fixed ; one) reading sidereal time — At by its two 
fixed verniers, and the upper being set to sidereal time by its fixed verniers, two differ- 
ential verniers read true Hts. They are divided on an alloy of palladium and silver to 
l m , and the verniers read to I s . For the purpose of setting this differential circle, pins 
f, f \ f are inserted in it, by which it may be turned to its approximate position when the 
final adjustment is made by a delicate tangent motion. 
Plate VII. fig. 22 is a plane of the At clamp and clock sector removed from the axis ; 
scale 1-| inch to 1 foot. 
The sector driven from the clock by an endless screw is strung on a portion of the 
polar axis 7-| inches diameter, immediately above the hour-circles by the cast-iron piece 
A bored accurately to fit it ; to this are attached two steel tubular arms which carry on 
their extremity the gun-metal sector B, 5 feet radius ; these are strongly braced with 
cast-iron lattice trussings, which carry at 3-feet radius a square-threaded tangent screw 
a, communicating with the clamp C by the nut-and-link motion b, shown in larger scale 
and described in Plate VII. figs. 23 and 24. The clamp C, 20 inches diameter, is 
clamped or unclamped to the axis by half a turn of either of the screws c or c ' . When 
unclamped the instrument is free for motion in At by the quick-motion gearing, driven by 
the hand-wheel G (Plate XII. fig. 15). When clamped, the clockwork being in motion, 
the telescope follows the object. The final adjustment in At for bringing the object into 
the centre of the field of the telescope is effected by the differential screw a, actuated 
through the shafts e, by Hoos’s-joint handles attached to either d or d'. 
Plate VII. fig. 23 is a plan, and fig. 24 a vertical section of the parallel-motion 
arrangement for the tangent screw referred to above ; scale ^ size. 
Owing to the great sizes and weights of the masses to be moved, the ordinary con- 
structions extant were considered insufficient. In the arrangement as adopted the screw 
is supported in bearings at both ends. This involves the necessity of the nut moving in 
a straight line, while the arm which carries it describes an arc of a circle. This is pro- 
vided for by the arrangement shown in the figures, a a is a portion of the screw (which 
is purposely made of considerable length) working in the nut b b. This nut has a pair 
of trunnions towards one end, A A, working in the inner frame c c, which has itself a pair 
of trunnions, B B, working in the outer frame d d, which is attached to the arm of the 
clamp. At e is seen a counter-nut to take away any loss that may be occasioned by the 
wearing of the screw. This nut is cut into teeth on its edge, and is kept in its place by 
