LATELY ERECTED AT THE ROYAL OBSERVATORY. 



211 



property may at some future period be applied with advantage in investigations made 

 with moveable zenith instruments. 



For the present purpose it is only essential to remark, that for the investigation of 

 these small equations it is by no means required to have determined either the exact 

 zenith distance of either star, or the exact difference of their zenith distances, or the 

 absolute magnitude of this subsidiary angle, its variation from time to time being the 

 only important object of research. 



It is the more necessary to keep this in mind, as occasions may arise when it may 

 be found advisable to omit altogether the investigation of the real zenith distance, 

 and to confine the attention to the variations only of this small angle. 



The improving performance of the zenith telescope leads me to hope that ere long 

 I shall be able to illustrate the principle of the method of observation I have de- 

 scribed, by a series of observations made with it ; and should I not be disappointed in 

 my expectations, the instrument will, I am of opinion, be found to rank among the 

 most important in the Observatory. 



The principles above explained apply with equal correctness to altitude and azimuth 

 instruments, and may be advantageously adopted in their use. 



Example. 



