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as to present great obstacles to its motion. The vast surface 

 exposed to the action of wind must have made it unsteady ; 

 and its durability could not be great. Lord Rosse, therefore, 

 determined to confine the range of observation to the vicinity 

 of the meridian. There the stars are at their greatest altitudes, 

 and atmospheric influences affect them least ; their places can 

 be determined with most accuracy, and an equatorial move- 

 ment, so essential to micrometer measures, can be easily 

 obtained. With such optical power there will never be a 

 scarcity of objects for examination ; and the restriction will 

 only be felt in the case of planetary bodies. The base of the 

 actual mounting is a very massive joint of cast iron ; its lower 

 axis permitting motion in the meridian plane, its upper in a di- 

 rection perpendicular to that circle. On this is firmly bolted a 

 cubical wooden chamber, about eight feet wide, in which the 

 speculum is placed, one of its sides opening for the purpose. This 

 again carries the tube, which when vertical and viewed from the 

 interior of the chamber, is more like one of the old round towers 

 than any more ordinary object of comparison. It is fifty feet 

 long, eight feet in diameter in the middle, but tapering to seven 

 at the extremities : it is made of deal staves an inch thick, 

 hooped with strong iron clamp-rings, and secured from collapse 

 by iron diaphragms ; and carrying at its upper extremity the 

 apparatus of the Newtonian small mirror, which, from its great 

 weight and bulk requires to be counterpoised. The telescope 

 is moved in declination by a strong chain cable attached to its 

 top and passing over a pulley fixed at a proper height to the 

 north, down to a windlass on the ground which is wrought by 

 two workmen. East and west, near the top of the piers, large 

 iron pulleys are fixed, having free movement in azimuth, so 

 that their planes may always be in those of the traction : chains 

 suspending the counterpoise weights pass over these to the 

 sides of the tube. The weights, however, are constrained to 

 descend in quadrants of circles by chain guys attached to the 

 frame which bears the declination pulley. It is easily seen 



