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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 nebulae, 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 for a star at the equator ; 

 and Lord Rosse intends to eiFect 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 it is 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 



