The Earth. 339 



eclipses of the satellites of Jupiter show the hour 

 by the clock of London at the time when they 

 are observed ; the difference, then, between the 

 times observed at the different places, will give 

 the difference of longitude. This is the reason 

 why a clock or time-piece which does not vary 

 at all, and which is set to the time of the place 

 from which a vessel sails, will always serve to 

 show the difference of time between whatever 

 place it may be at, and that of the place which 

 it has left, and consequently will indicate the 

 longitude, provided it goes accurately. 



To render this matter still more familiar, a* 

 the sun appears to move uniformly round the 

 earth, and to describe a circle, which contains 

 360 degrees, in twenty-four hours, he will of 

 course move through an arc of 15 degrees in an 

 hour. When it is noon, therefore, at London 

 and at all other places which lie under the same 

 meridian, it will be one o'clock in the afternoon 

 at all those places which 1 lie under the meridian 

 15 degrees to the east of that of London ; and 

 eleven o'clock in the morning, at all those places 

 which lie under the meridian 15 degrees to the 

 west of that of London. If the distance of the 

 meridians are 30 degrees, it will make two hours 

 difference in the time ; if 45 degrees, three hours, 

 &c., reckoning according to the situation of the 

 places. 



From these circumstances you will readily ob- 

 serve, that as places differ in longitude, or are 



