APPLICATION OF THE ELECTRIC TELEGRAPH. 365 



the time required for the star to describe this portion of 

 its path ; and the observed time is easily converted into 

 arc of a great circle. If a third star enters the field at 

 M, and crosses the wire D E at N, and F G at O, then 

 C N is the difference of declination of the first and third 

 stars ; and in the same manner, by observing the transits 

 of any number of stars over the wires D E and F G, in 

 the same position of the telescope, we shall obtain their 

 differences of declination as well as of right ascension. 

 In order to diminish the errors of observation, we intro- 

 duce a large number of inclined wires, at intervals of two 

 or three seconds from each other, as well as a large num- 

 ber of vertical wires ; and the times of transit over each 

 system of wires are recorded by electro-magnetism. 



This method is well adapted to the construction of a 

 catalogue of stars, where it is proposed to record the 

 position of every star within the range of the telescope. 

 For this purpose the telescope is firmly clamped, and re- 

 mains fixed in its position during the observations of an 

 entire evening or night, while the observer, sitting with his 

 eye at the telescope, has but to press his finger upon a key 

 at the instant a star is seen to pass each wire of the two sys- 

 tems already mentioned. This mode of observation has been 

 practiced at the Washington observatory since 1849. The 

 wires for right ascension are 35 in number, and are divided 

 into groups or fascicles of five each, the interval between 

 two wires being from two to three seconds. To complete 

 a set of observations on any one fascicle requires only 

 from eight to ten seconds. The wires for differences of 



