30 Astronomy and Geology compared. PT. i. 



error which may arise from the motion of the ship. 

 It is evident from the foregoing that, as the deter- 

 mination of the longitude is arrived at by ascertain- 

 ing the time at two places simultaneously, the 

 astronomical means of effecting this is by converting 

 the heavens into one great clock, from which the 

 observer can read the hour as from a dial-plate ; 

 and in order to accomplish this, three things are 

 necessary. First, to ascertain the exact time at the 

 place of observation; secondly, to know, by the 

 position of the Sun, Moon, and stars, the corre- 

 sponding time at Greenwich; and, thirdly, that the 

 motions of these bodies should be so uniform and 

 regular as to enable the astronomer to calculate 

 beforehand what will be their exact apparent dis- 

 tances from each other at every hour of the twenty- 

 four. Now it is this last calculation which is made 

 in the Nautical Almanac for every year, and which 

 is in constant use in navigation, and which furnishes 

 a convincing proof of two astronomical facts. First, 

 the wonderful accuracy and regularity of the move- 

 ments of the Earth and its attendant Moon through- 

 out the whole of its annual revolution. Secondly, 

 the thorough mastery which astronomical science 

 must have obtained over the knowledge requisite 



