343 PHYSIOGRAPHY. [chap. 



might mean any hour in the day or night. The reason of 

 this is that day and night depend upon the sun, and the sun 

 does not keep sidereal time. In the first place, the interval 

 between the time when the shadow on a sun-dial marks noon 

 on one day, and the time it marks noon on the next day, is 

 always more than 86,164 seconds; and, in the second place, 

 the difference is not always the same, but is sometimes more 

 and sometimes less. If the sun-dial were a clock, in fact, 

 we should say that it did not go very well ; and the only way 

 of making a good clock go, in such a manner as to show 

 twelve at noon by the sun, or thereabouts, every day, is to' 

 strike an average of all the irregularities of the sun-dial, 

 and add thi.s average to the number of seconds, which 

 would be marked by the revolution of the hand of a star- 

 clock in the course of a day. 



This average is 236 seconds, which, added to 86,164, 

 gives the 86,400 seconds which compose the 24 hours, 

 or mean solar day, of civil time. For convenience' sake 

 these are counted, not by one revolution, but by two, of the 

 hour-hand of an ordinary clock ; and thus the XII. on our 

 clocks shows, very nearly, the midday and midnight as 

 determined by the sun's crossing the meridian. The coin- 

 cidence of twelve by the clock with noon by the sun-dial, how- 

 ever, is exact only four times in the year ; at the intervening 

 periods, the dial is either faster or slower than the clock. 



Since the earth's figure is nearly spherical, it follows, that 

 different points on the earth's surface must move, during the 

 daily rotation, with different velocities. Any point on the 

 line of the equator will describe a circle equal to the circum- 

 ference of the earth. The earth's circumference is about 

 24,000 miles ; and, as the rotation is effected in nearly twenty- 

 four hours, the velocity of its equatorial region must be 



