OUR CHIEF TIME-PIECE LOSING TIME. 31 



It is determined, we find, as a certain fraction of the 

 length of a pendulum vibrating seconds in the latitude 

 of London. A second, we know, is a certain portion of 

 a mean solar day, and is practically determined by a 

 reference to what is called a sidereal day the interval, 

 namely, between the successive passages by the same 

 star of the celestial meridian of any fixed place. This 

 interval is assumed to be constant, and it has, indeed, 

 been described as the ' one constant element ' known to 

 astronomers. 



We find, then, that there is a connection, and a 

 very important connection, between the motion of 

 the stars and our measures, not merely of value, but 

 of weight, length, volume, and time. In fact, our 

 whole system of weights and measures is founded on 

 the apparent 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 outwards 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, measuring, and valuing terrestrial objects 

 with an exactitude which is at present beyond our 

 wants. 



The earth, then, is our * chief time-piece,' and it is 

 of the correctness of this giant clock that I am now to 

 speak. 



But how can we test a time-piece whose motions 



