OUR CHIEF TIME-PIECE LOSING TIME. 39 



we select to regulate every other time-piece? If a 

 man sets his watch every morning by the clock at 

 Westminster, it is clearly impossible for him to test 

 the accuracy 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 illus- 

 tration, 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 time-pieces cannot be employed to detect 

 slow variations, if any such exist, in the earth's 

 rotation-period. 



Sir William Herschel, who early saw the import- 

 ance of the subject, suggested another method. Some 

 of the planets rotate in such a manner, and bear 

 such distinct marks upon their surface, that it is pos- 

 sible, by a series of observations extending over a 

 long interval of time, to determine the length of 

 their rotation-period within a second or two. Sup- 

 posing their rotation uniform, we at once obtain an 

 accurate measure of time. Supposing their rotation 

 not uniform, we obtain (1) a hint of the kind of 

 change we are looking for; and (2), by the comparison 

 of two or more planets, the means of guessing how 

 the variation is to be distributed between the observed 

 planets and our own earth. 



Unfortunately, it turned out that Jupiter, one of 

 the planets from which Herschel expected most, does 



