xiv.J UNITS AND STANDARDS OF MEASUREMENT. 311 



standard of time than the earth in its annual motion. 

 The greatly superior mass of Jupiter and its satellites, and 

 their greater distance from the sun, may render the 

 electrical dissipation of energy less considerable than in 

 the case of the earth. But the choice of the best measure 

 will always be an open one, and whatever moving body 

 we choose may ultimately be shown to be subject to 

 disturbing forces. 



The pendulum, although so admirable an instrument for 

 subdivision of time, fails as a standard ; foi though the 

 same pendulum affected by the same force of gravity per- 

 forms equal vibrations in equal times, yet the slightest 

 change in the form or weight of the pendulum, the least 

 corrosion of any part, or the most minute displacement of 

 the point of suspension, falsifies the results, and there enter 

 many other difficult questions of temperature, friction, 

 resistance, length of vibration, &c. 



Thomson and Tait are of opinion l that the ultimate 

 standard of chronometry must be founded on the physical 

 properties of some body of more constant character than 

 the earth ; for instance, a carefully arranged metallic 

 spring, hermetically sealed in an exhausted glass vessel. 

 But it is hard to see how we can be sure that the dimen- 

 sions and elasticity of a piece of wrought metal will 

 remain perfectly unchanged for the few millions of years 

 contemplated by them. A nearly perfect gas, like 

 hydrogen, is perhaps the only kind of substance in the 

 unchanged elasticity of which we could have confidence. 

 Moreover, it is difficult to perceive how the undulations of 

 such a spring could be observed with the requisite 

 accuracy. Mere recently Professor Clerk Maxwell has 

 made the novel suggestion, discussed in a subsequent 

 section, that undulations of light in vacua would form the 

 most universal standard of reference, both as regards time 

 and space. According to this system the unit of time 

 would be the time occupied by one vibration of the par- 

 ticular kind of light whose wave length is taken as the 

 unit of length. 



1 The Elements of Natural Philosophy, part i. p. 119. 



