232 HISTOKY OF ASTRONOMY. 



of parallel wires, the intervals of which have been care- 

 fully determined. Thus, in whatever part of the field a 

 star appears, a micrometer wire is close at hand, and the 

 star is bisected by the nearest wire, while the time at 

 which it passes the several vertical wires is also noted. 

 The number of the bisecting wire, and the reading of the 

 micrometer being now entered, the observation is complete. 



The observer thus keeps his eye at the telescope for 

 hours at a time, and, under favorable circumstances, can 

 observe with ease two or three hundred stars during the 

 night. The transit instrument, by means of a mi- 

 crometer moving in altitude, is converted into a difference 

 of declination instrument, and occupies the adjoining belt 

 above the mural, the two instruments being so set that 

 ten minutes of declination are common to the field of 

 both. The meridian circle in the same way occupies the 

 belt below the mural. The next night the instruments 

 change places, and go over.. the same ground, i. e. t the 

 meridian circle covers the same belt to-night which on 

 the former night was swept by the mural. The two lists 

 are immediately compared, and should it appear that any 

 of the stars have changed their position, the large equa- 

 torial is put in pursuit, to see whether they are fixed 

 stars or not. 



This great work contemplates the examination of every 

 star down to the tenth magnitude in the entire visible 

 heavens ; and while it looks to the discovery of new 

 planets and unknown stars, it also aims to detect the dis- 

 appearance of any stars found in existing catalogues. 



