218 



Mr. A. Le Sueur on the 



[Feb. 17, 



for a laboratory. Tbe moveable roof is 40 feet long, and runs on rails 

 laid tbe whole length of tbe walls ; the telescope-room may therefore be 

 completely covered in, and as completely uncovered when required, the 

 roof in the latter case resting on the south building, which on that account 

 has a very low permanent roof. 



The roof is constructed of six triangular wrought-iron principals, cross- 

 braced, which abut at each side on a broad horizontal plate formed of two 

 parallel lengths of stout angle-iron, connected at various points by iron 

 bands ; for additional strength, a broad vertical plate is bolted to the 

 outer angle-iron piece. There are four pairs of wheels, 26 inches in dia- 

 meter, flanched on the inside ; these lie along the middle of the horizontal 

 plate, the journals being bolted to the angle-iron pieces which form the 

 plate. 



The roof is covered with galvanized corrugated iron ; it is therefore on 

 the whole a somewhat heavy affair. The mechanical arrangements for 

 moving are, however, simple and effective ; a stout iron shaft runs across 

 the building, and gears by wheel and pinion on the axles of the two south 

 end wheels ; to this shaft is fixed a spoked hand-wheel, by means of 

 which the operator readily sets the roof in motion, and standing on 

 a small platform connected therewith, is himself carried along at the same 

 time. 



The design of the roof is due to Mr. Merrett, of the Works depart- 

 ment. On the whole, there is much to be said in favour of this rectan- 

 gular form of roof : the temperature even in this climate frequently de- 

 scends too low to be pleasant ; but the occasional bodily inconvenience pro- 

 duced thereby is more than counterbalanced by perfect freedom to the 

 observer, and the gratification of knowing that the instrument is in the best 

 possible conditions for satisfactory performance. Only one really serious 

 annoyance have I found connected with complete exposure ; I allude to 

 occasional heavy dew rendering it almost impracticable to use the sketch- 

 ing and other papers, the speculum meanwhile remaining free from de- 

 posit if precaution is taken not to work at too great an altitude. 



The telescope, when housed, lies meridionally on the east side of the 

 pier, and nearly in a horizontal direction, provision having been made to 

 prevent the tube being lowered beyond a certain small inclination. 



The piers are in keeping with the massiveness of the instrument ; they 

 are constructed of large, not to say huge blocks of basalt axed to a fine 

 surface, altogether a substantial and beautiful piece of work. 



The height of the walls with reference to the piers is such that very 

 little of the sky range is curtailed. The north wall cuts off objects having a 

 lesser altitude than about 10°. When resting on the east or west walls the 

 telescope is nearly horizontal ; in both these directions trees interfere, 

 especially on the west side, where the ground rises. This curtailment 

 will probably be a matter of very small importance, as with a four-feet 



