SICT. XII. DIVISIONS OF TIME. 79 



the Nautical Almanac, sometimes amounting to as much 

 as sixteen minutes. The apparent and mean time coin- 

 cide four times in the year ; when the sun's daily mo- 

 tion in right ascension is equal to 59' &" 33 in a mean 

 solar day, which happens about the 16th of April, the 

 16th of June, the 1st of September, and the 25th of 

 December. 



The astronomical day begins at noon, but in common 

 reckoning the day begins at midnight. In England it is 

 divided into twenty-four hours, which are counted by 

 twelve and twelve ; but in France astronomers, adopting 

 the decimal division, divide the day into ten hours, the 

 hour into one hundred minutes, and the minute into a 

 hundred seconds, because of the facility in computation, 

 and in conformity with then* decimal system of weights 

 and measures. This subdivision is not now used in 

 common life, nor has it been adopted in any other 

 country ; and although some scientific writers in France 

 still employ that division of time, the custom is begin- 

 ning to wear out. At one period during the French 

 revolution, the clock in the gardens of the Tuileries was 

 regulated to show decimal time. The mean length of 

 the day, though accurately determined, is not sufficient 

 for the purposes either of astronomy or civil life. The 

 tropical or civil year of 365 d 5 U 48 m 49 8 -7, which is the 

 time elapsed between the consecutive returns of theun 

 to the mean equinoxes or solstices, including all the 

 changes of the seasons, is a natural cycle peculiarly 

 suited for a measure of duration. It is estimated from 

 the winter solstice, the middle of the long annual night 

 under the north pole. But although the length of the 

 civil year is pointed out by nature as a measure of long 

 periods, the incommensurability that exists between the 

 length of the day and the revolution of the sun, renders 

 it difficult to adjust the estimation of both in whole num- 

 bers. If the revolution of the sun were accomplished 

 in 365 days, all the years would be of precisely the same 

 number of days, and would begin and end with the sun 

 at the same point of the ecliptic. But as the sun's revo- 

 lution includes the fraction of a day, a civil year and a 

 revolution of the sun have not the same duration. Since 

 the fraction is nearly the fourth of a day, in four years 



