28 Astronomy and Geology compared. PT. i. 



distance in degrees and miles east or west of that 

 meridian. Say, for instance, that when it is twelve 

 o'clock at Greenwich, it is eleven o'clock at some 

 point to the west of Greenwich, we then know that 

 the distance is 15; or if, on the contrary, it is one 

 o'clock at the place of observation and it is twelve 

 o'clock at Greenwich, we know that we must be 15 

 to the east of Greenwich. But, in order to institute 

 this comparison, it is necessary to possess the means 

 of ascertaining the exact time at Greenwich and at 

 the place of observation simultaneously. The time 

 at the place of observation is obtained by certain 

 astronomical observations, which it is not necessary 

 now to particularize. The time at Greenwich is 

 calculated by two methods. The first and easiest is 

 by a chronometer set to Greenwich time, and there- 

 fore indicating the hour at that place, wherever the 

 chronometer is taken. The great improvements 

 which have been made in the construction of chro- 

 nometers has rendered this simple method one of 

 considerable accuracy. It is exceedingly valuable to 

 navigators in conjunction with that more scientific 

 process, by lunar observation, as verifying a cor- 

 rection of any error in the reckoning, and also as 

 filling up the blank occasioned by any interruption 



