ASTRONOMY. 63 



with the time, as reckoned there, it will give the 

 difference of longitude. 



b. Suppose at the first place, by repeated observa- 

 tions, the star Sirius was found to pass the meri- 

 dian at 10 h 24 m 35sec by the watch, and at the 

 other at 8 h 16 m 22sec by t ] ie same wa t c h ; the dif- 

 ference, 2h 8 m 1 3 s c 5 converted into degrees, gives 

 32 3' 15" for the difference of longitude, by 

 which the first of the two places is east of the se- 

 cond. 



67. In like manner, any phenomenon, the be- 

 ginning or end of which is seen at the same in- 

 stant by observers under different meridians, af- 

 fords the means of determining the difference 



of longitude. 

 G 



For, by this means, the difference of the reckoning 

 at the two places is ascertained just as by the chro- 

 nometer. 



Many of the phenomena of the heavens, as will be 

 afterwards explained, serve as signals of this 

 kind. 



Such signals may also be given on the earth, by the 

 sudden kindling or extinguishing of lights on some 

 elevated station, from which they may be seen at 

 considerable distances. 



The observation of the convergency of the meridians, 

 is also another way of determining the longitude, 

 as already observed. To it, and the two now men- 

 tioned, 



