OUR CHIEF TIMEPIECE LOSING TIME. 47 



system of weights and measures is founded on the ap- 

 parent diurnal motion of the sidereal system, that is, 

 on the real diurnal rotation of the earth. We may 

 look on the meridian-plane in which the great transit- 

 telescope of the Greenwich Observatory is made to 

 swing, as the gigantic hand of a mighty dial, a hand 

 which, extending outward among the stars, traces out 

 for us, by its motion among them, the exact progress 

 of time, and so gives us the means of weighing, meas- 

 uring, and valuing terrestrial objects with an exactitude 

 which is at present leyond our wants. 



The earth, then, is our " chief timepiece," and it is 

 of the correctness of this giant clock that we are now 

 to speak. 



But how can we test a timepiece whose motions 

 we select to regulate every other timepiece ? If a man 

 sets his watch every morning by the clock at West- 

 minster, it is clearly impossible for him to test the ac- 

 curacy of that clock by the motions of his watch. It 

 would, indeed, be possible to detect any gross change 

 of rate ; but, for the purpose of illustration, I assume, 

 what is indeed the case, that the clock is very accurate, 

 and, therefore, that minute errors only are to be looked 

 for even in long intervals of time. And, just as the 

 watch set by a clock cannot be made use of to test the 

 clock for small errors, so our best timepieces cannot be 

 employed to detect slow variations, if any such, exist, 

 in the earth's rotation-period. 



