38 LIGHT SCIENCE FOR LEISURE HOURS. 



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 we are now 

 to speak. 



But how can we test a time-piece whose motions 



