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the place of an hour circle for finding objects, for which it is 
quite sufficient, except that the strong light required to set it 
disturbs the repose of the eye. The elder Herschel has not 
in the least exaggerated the importance of this when faint ob- 
jects, especially nebula, are to be examined; and a better 
contrivance is to be applied. The rack being perpendicular 
to the meridian, gives a motion not strictly equatorial, but 
easily made so: had the declination pulley been in a parallel 
to the earth’s axis, passing through the great joint, and had 
this latter been itself equatorial, this would have been the case ; 
but the deviation is easily corrected by the addition of a second 
pulley altering the direction of the chain. Its range is half 
an hour on each side of the meridian fora star at the equator ; 
and Lord Rosse intends to effect it by clock-work, as is now 
generally the case in large equatorials; though the problem is 
much more difficult than in those instruments. 
The western pier supports the stairs and galleries destined 
to the observers. Up to 42° of altitude is commanded by the 
first of them: a strong and light prismatic framing slides be- 
tween two ladders attached to the southern faces of the piers: 
it is counterpoised and is raised to any required position by a 
windlass ; its upper plane affords support for a railway on 
which the observing gallery moves about twenty-four feet east 
and west, two of its wheels being turned by a winch near the 
observer. Three other galleries in succession reach to 5° below 
the pole; these are each carried by two beams which run be- 
tween pairs of grooved wheels, and are drawn forward, when they 
are turned, by a mechanism of singular elegance. These are able 
to hold twelve people, but one man can easily work them; and 
though itis rather startling to a person who finds himself sus- 
pended over a chasm sixty feet deep, without more than a specu- 
lative acquaintance with the properties of trussed beams, all is 
perfectly safe. Every bearing part has been proved to ten times 
its utmost probable load, and the doors of the galleries open 
inward, and are kept close by springs. From this point too is 
