454 Description of the Great Southern Telescope. [ Oct., 
difficulties were not disposed of, would be a complete bar in the way of 
mounting them equatorially. ‘Two things are essentially necessary. 
1st. A system of back supports on which the speculum may le 
in a state free from strain of any kind, as if in fact it were floating 
in mercury. 
2nd. A system of lateral support which will preserve the mirror 
free from strain when turned off the zenith, and will not constrain 
any slight movement of the speculum. 
Now as regards the first condition, nothing could fulfil it better 
than the system introduced long ago by Mr. Grubb, which was 
made use of on such a grand scale by the late Lord Rosse as well 
as by others, and this system therefore with some modification was 
adopted. 
To understand it, suppose the speculum to be divided into forty- 
eight portions as in Fig. 7, each of them being exactly equal in area 
and consequently in weight. 
Now if the centre of gravity 
of each of these pieces rested 
on points which would bear 
up with a force = the weight 
of each segmental piece, it is 
evident that there would be 
no strain in the mass from 
segment to segment. This is 
exactly what is accomplished 
by this system; in fact, if 
when the speculum is resting 
on these supports it could be 
divided up into segments 
corresponding to those lines 
they would have no inclination 
to leave their places, showing 
a perfect absence of strain across those lines. Suppose now the 
points representing the centres of gravity of these segments were 
supported on levers and triangles, so as to couple them together as 
at A, Fig. 8, and each of these couplings to be supported from a 
point a, representing the centre of gravity of the sum of the 
segments supported by that particular couple, and it is evident that 
there can be no strain between the components of these couples. 
Again, let these points a be coupled together by the system shown 
at B, Fig. 8, and their centres of gravity b coupled as at C, and 
it is evident that the whole weight of the speculum ultimately con- 
densed by this system into these points is supported on forty-eight 
points of equal support, being the centres of gravity of the forty- 
eight segments at Fig. 7. In Fig. 9 is seen the whole system 
complete. It consists of three screws passing through the back of 
