121 



it stands.* Here also there were five or six failures, till Lord 

 Rosse noticed that there was a diflSculty in keeping the spe- 

 culum properly coated with peroxide of iron ; and the disturb- 

 ing cause was soon detected. The pitch of which the polisher 

 is made possesses the requisite consistence only at the tempe- 

 rature of 55°. At that time, however, it was below freezing, 

 and it was necessary to warm the laboratory by stoves. The 

 air of that room, therefore, became drier, and evaporated the 

 moisture from the speculum and polisher too rapidly. On exa- 

 mining this with the wet-bulb hygrometer, they found in one 

 instance 17° difference. This was remedied by a jet of steam 

 from a small pipe connected with the boiler of a steam engine, 

 which was regulated so as to keep the air nearly saturated with 

 moisture ; and at once all difficulty was removed. The specu- 

 lum defined the dial-mark quite sharply with a power of 3800, 

 and, when placed in its tube, left nothing to be desired. The 

 six feet was polished with equal success next day, February 16. 

 Originally the movement in right ascension was given 

 through a handle moved by the observer. This was found in- 

 convenient; and the apparatus is connected with a drum below, 

 moved by a workman. It is found that this will afford a ready 

 means of mechanical movement by clock-work, which is now 

 in hands. The arrangement chosen by Lord Rosse is a gi- 

 gantic metronome, the pendulum of which will carry a gradua- 



• If a lucid point be at the distance d from a parabolic mirror, whose 

 focal length =/, its image is formed at a distance from the principal focus 

 for central rays, 



z=-^ — ; 

 d-f 



for a zone whose distance from the axis -y : this distance is further increased 



by 



^ d-fV^Qp]- 



Z can be measured for different zones, and if it have this value, the speculum 

 must be parabolic. 



VOL. IV. L 



