540 Royal Astronomical Society. 



south of the fixed support, are brought successively into action as 

 the telescope is raised. There is then only a comparatively small 

 and very manageable tendency of the telescope towards the south, 

 and this is supported by a light chain which passes over a pulley on 

 a bar connecting the horns before mentioned (the pulley being in 

 the direction of a polar axis passing through the lower universal 

 joint, and the motion of the telescope, therefore, for a given length 

 of the chain, being equatoreal), and this chain is shortened or 

 lengthened, and the telescope is thereby raised or depressed, by a 

 windlass a little way north of the fixed support. Upon the inner 

 face of the eastern pier is an iron arc of a circle, upon which slides 

 a runner connected with a rod that passes through a frame on the 

 telescope tube and near to its mouth, and is there racked for working 

 with a pinion : by the movement of this pinion the distance of the 

 telescope from the pier is altered, and thus a motion in hour- angle 

 is given. At the south ends of the piers there are strong ladders, 

 upon which (assisted by counterpoises) there slides a stage ; upon 

 which stage a small observing-box travels east and west : this is 

 used for observing, so long as the mouth of the telescope is below 

 the end of the pier. For greater elevations, the top of the western 

 pier being shaped by slopes so as to approximate to a circular arc, 

 there are mounted upon it curved galleries, which are carried by 

 beams that run above and below pulleys fixed to the top of the pier ; 

 and the galleries are carried out by rack-and-pinion work, to ap- 

 proach the side of the telescope. It is intended to give the power 

 of observing as far north as the pole ; but at present the galleries 

 extend only to the zenith. The telescope is Newtonian, the minor 

 axis of the small mirror being about six inches, and the observer 

 looks into the side of the tube. 



Mr. Lassell's tube is of sheet-iron ; and this tube is not carried 

 immediately by the mounting, but is inserted in a long box of cast 

 iron, in which it can be turned round its own axis. This movement 

 is necessary to place the eyepiece exactly in the same side-position 

 in all directions of the telescope, and also to cause the edgewise 

 support of the mirror to act always in the same way. The long box 

 is mounted equatoreally, the polar axis turning in two bearings be- 

 low the declination axis, and carrying an hour-circle, upon which 

 are fixed two supports, in which turn the two pivots of the declina- 

 tion axis of the long box. The telescope is Newtonian, the eye- 

 tube being in one side ; but the smaller dimensions of the small 

 mirror (a diameter of two inches only being required) enable Mr. 

 Lassell to use the reflexion at the internal surface of a glass prism, 

 by which much more light is reflected than by a metallic reflector. 

 At first much annoyance was caused by the deposition of dew on 

 the glass, but this was remedied by attaching to it a case carrying a 

 small piece of heated lead ; and when proper attention is given to 

 the inclosure of the lead, no inconvenience is sustained from the 

 efi^ect of the hot metal in disturbing the air in the tube of the tele- 

 scope. The whole is covered by a revolving dome thirty feet in 

 diameter, and the observer is mounted for observation on a stage 

 which is carried by the dome. 



