602 Scientific Proceedings, Royal Dublin Society. 



brought exactly at right angles to the shaft which carries the fork 

 •on which these trunnions revolve. By a second series the shaft 

 itself was brought at right angles with the declination axis ; and 

 by a third series the declination axis was brought at right angles 

 with the polar axis. 



The polar axis was then set to the correct inclination by an 

 "inclinometer." Thus the whole equatorial part was adjusted 

 without aid of any celestial observations, in every respect except 

 azimuth, which can only be found by observation. Then a spe- 

 cially prepared plumb bob, hung from centre of declination axis, 

 gave the exact position of the centre of rotation of the mirror 

 frame ; and finally (the slipping piece having been removed from 

 its fork) the fork was raised until its centres coincided with the 

 horizontal axis of the mirror. Bushings with small electrical 

 holes were fitted into the bearings of mirror and of fork, and a 

 fine silver wire stretched through all four holes ; this insured that 

 the distance from declination axis to mirror axis was precisely 

 similar to that from declination axis to centre of motion of slipping 

 piece, and this completed the adjustment so far as it is possible to 

 do it mechanically. 



I am indebted to the Editors of " Engineering " for the illus- 

 trations to this Paper. 



