WKICillTS AND MEASURES. 127 



immutable unit. But it is not the unit that is used, for it is 

 not convenient. 



Owing to the movement of earth in orbit it comes to pass 

 that supposing the sun and a star to be on the meridian 

 together on a given day, the sun will, on the next day, come 

 to the meridian some time behind the star. The difference is 

 rather less than four minutes and varies according to various 

 circumstances. For convenience it is imagined constant, and a 

 "mean solar day," in reality 86,635.96 mean solar seconds long, 

 is called an even twenty-four hours, and the 86,400th part of 

 this is taken as the second of common life. It is one sidereal 

 second and two hundred and seventy-two one hundred thou- 

 sandths of a sidereal second (1.00272). 



This second is told off by one beat of the pendulum spoken 

 of above. It is the second of common life, of clocks and 

 watches — the standard second, permanent, invariable, because 

 it bears a known proportion to a quantity which is permanent 

 and invariable. 



The original unit, then, is the mean sidereal second, or 

 86,400th part of a mean sidereal revolution of the earth on its 

 axis. From this is derived the mean solar second, the invari- 

 able standard unit of time ; which gives us through the pen- 

 dulum the standard of length, and through length the stan- 

 dard of weight. 



The existence, moreover, of any record, however rude, of 

 the lapse of time, implies some knowledge of the heavenly 

 bodies and their motions, since by them only can time be 

 measured. 



In view of this, it seems not too much to say that human 

 transactions are regulated by the stars in their courses. 



But the stars move in their courses as God bids them. 



