TTIK SFNDIAI. AXD TUV. CLOCK. I05 



expression in its fullest sense. This space of time is divided into 

 twenty-four equal parts or hours ; each hour into sixty minutes 

 and each minute into sixty seconds ; in all 86,400 seconds. 



The second, then, is the eighty-six thousand four hun- 

 dredth part of the earth's mean sidereal revolution ; and it is 

 an immutable unit. 



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



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

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

 at the same time on a given day the sun will on the next day 

 come to the meridian some little time behind the star. The 

 difference is rather less than four minutes, and varies accord- 

 ing to various circumstances. For convenience it is imagined 

 constant and a "mean solar day", in reality 86,636.96 sec- 

 onds long is called an even 24 hours. 



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 thousandths of a sidereal second (1.00272). 

 This second is told off by one beat of a pendulum — in vacuo 

 — 62° Fahr. ^ — latitude of London — sea level — and thirty- 

 nine inches and thirteen-hundred and ninety-three ten thous- 

 andths of an inch long. It is the second of common life, of 

 clocks and watches ; the standard second ; permanent, invar- 

 iable ; because it bears a known proportion to a quantity 

 which is permanent and invariable. 



The real sun is sometimes before the mean sun in yearly 

 course and sometimes behind. The more he is before the real 

 sun the greater his retardation, as it is called, in coming to the 

 meridian, and the more he is behind the more he precedes the 

 mean sun in coming to the meridian. It follows that the two 

 are together at certain times. And it is clear that they are 

 together whenever the real sun, being behind, is so accelerated 

 by the causes set forth as to overtake the mean sun ; and 

 again when having thus overpassed the mean sun he is so 

 retarded as to have lost his lead and fall behind. 



The expressions "sun's yearly motion" and "earth's 



