450 Description of the Great Southern Telescope. [ Oct., 
friction—the polar axis is laid hold of by a very peculiar steel link 
chain, so constructed that the links can twist a small quantity on 
each other without producing friction, which chain is held up by a 
trussed lever of the proportion of about 8 to 1, acted upon by 
weights at the back of the pier amounting to about half-a-ton, so as 
to relieve about 4 tons of end pressure. 
So effective is this arrangement, that a force of 5 pounds ata 
point 20 feet from the centre of motion, is sufficient to move this 
mass of 8 tons. 
The arrangement for the relief of 
Fic. 2. the friction in the declination axis is 
necessarily much less simple. It will 
readily be seen by an inspection of 
Fig. 2, which we may suppose to repre- 
sent the end of the declination axis, and 
which for simplicity is drawn for a lati- 
tude of 45°, P A being the direction of 
the polar axis, that if we place the axis 
on Ys at the points say a and b, when 
we reverse the instrument to the other 
side of the meridian these bearing-points 
will be at respectively ¢ and d, and the 
axis would roll out of its bearings; or 
if we put the Ys at mean points e and f, 
all the weight would be on one or other of the Ys, as the instru- 
ment is reversed from one side of the meridian to the other, and it 
would be quite impossible to relieve any portion of the friction by 
apparatus similar to that used for the polar axis, for what would 
be right on one side would be quite wrong on the other. Up to 
the present time this has been the great objection to this form 
of equatorial; for as explained above, the axis had necessarily 
to be put into the entire collared bearings without any possible 
relief of friction, the force necessary to move the telescope, if of any 
considerable size, became so great as to oblige the constructor to 
make the bearings of the declination axis very small, and conse- 
quently rendered the support of the telescope weak and unsteady. 
Only in 'this telescope, and in one other (also constructed by 
Mr. Grubb) which is now mounted at Dunsink Observatory, near 
Dublin, the object glass being presented by the late Sir James 
South to the University of Dublin, has this difficulty, one of the 
very few objections to this form of equatorial, been overcome. 
Referring to Fig. 2, suppose the weight W, acting perpendicu- 
larly, to be resolved into two forces, one, P, acting parallel and the 
other, R, at right angles to the polar axis. The first, P, inasmuch 
as it is parallel to, and along the axis, is constant in its direction to 
that axis as it turns round. The second, KR, varies in its direction 
