1868. ] Description of the Great Southern Telescope. 449 
massive wedge-shaped prisms carrying gun-metal blocks, bearing 
the axis, and which are acted upon by screws, so that motion can 
be given to them in a horizontal direction. (See Fig. 1.) 
This arrangement is for finally 
adjusting the axis into parallelism with iil 
the pole of the earth. It will be readily ches 
seen that by these two screws any 
required motion can be given: thus 
forcing both screws in or out equally, 
will give a vertical motion up or down ; 
while by advancing one screw in proportion as the other is with- 
drawn, a horizontal motion results in the direction of the retreating 
screw. 
The lower bearing rests in a block of gun-metal bored to fit 
the axis, and cut away for about 70° at the bottom to give it the 
property of a Y. The axis terminates in a flat-polished piece of 
chilled cast-iron, 5 inches diameter, bearing against a flat cushion 
of bell-metal, which cushion, of a spherical shape on its lower 
face, rests in a spherical cup. Therefore as the axis is adjusted 
into its proper direction by the screws in the upper bearing, the 
motion of the bell-metal cushion in its cup ensures a perfectly even 
bearing between it and the chilled iron and bell-metal surfaces. 
Now as the weight of the instrument as it rests on these bearings, 
of 12 and 6 inches diameter, amounts to about 8 tons, it follows 
that the friction, if not disposed of in some manner, would be so 
considerable as to render the instrument quite unmanageable ; but 
in all these bearings there is only about (th or ;1,th part of 
the weight really resting in the bearings themselves, whilst the 
remainder is supported by apparatus which reduces the friction 
to a minimum. In this manner are obtained great freedom of 
motion, less wear, and at the same time all the steadiness of the 
Y bearing; practically, in fact, more steadiness than if the whole 
weight were allowed to rest, for then it is found that an inclina- 
tion exists to ride up on the forward side. Accordingly, close 
above the lower bearing is placed a sector working on a hardened 
steel pin, and forced up by a screw and strong lamine of springs 
on which the axis rolls with a pressure of about 4 tons. Now 
as the radius of the sector is 27 inches, and the half diameter of 
the pin is =Z, inch, it follows that the friction is reduced in the 
proportion of =2,, or about 62 to 1. The same principle is 
carried out in the upper bearing, but here the weight not being so 
excessive, a roller of 8 inches diameter was thought sufficient, acted 
upon by a lever and weights hanging on the west side of the pier. 
In addition to this, to take off some of the pressure on the toe-pieces 
—more to prevent danger of biting than for the yet poe of paling 
I 
