Divisions of Time. 9 



counteract each other. There is also a fourth, 

 but much minuter modification, occasioned by 

 the precession of the equinoxes. On these four ac- 

 counts, and on established principles, astronomers 

 compute what are called equation tables, showing 

 when the sun is faster or slower than the clock, 

 or the difference between solar and mean time. 

 These are also given for the noon of every day 

 in the year, in the Nautical Almanac, the Con- 

 naissance des Terns, Bode's Jahrbuch, and 

 White's Ephemeris ; and for every Jifth day, in 

 the Ladies' and Gentleman's Diaries. 



Solar time, from these circumstances, can coin- 

 cide with mean time only at four periods of 

 the year, viz. about the 16th of April, the 15th 

 of June, the 31st of August, and the 25th of 

 December. On all the other days of the year 

 some inequality will be found, as shown by the 

 equation tables. 



\\Q have seen that the day consists of twenty- 

 four hours of mean time. But the term artificial 

 day is used to express the interval in which the 

 sun is above the horizon ; and the time during 

 which the sun is below the horizon, is called 

 night. The artificial day, it is well known, is 

 of different duration in different regions of the 

 globe, and at different seasons of the year. To 

 those who live under the equator it is exactly 

 twelve hours. At the pole the artificial day is of 

 six months' duration ; and at all those regions 



