364 THE PRINCIPLES OF SCIENCE. 



It is just possible that in the course of time, some other 

 body may be found to furnish a better 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 dissipa- 

 tion of energy less considerable even 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 assume, 

 may ultimately be shown to be subject to disturbing 

 forces. 



The pendulum, although so admirable an instrument 

 for subdivision of time, entirely fails as a standard ; for 

 though the same pendulum affected by the same force of 

 gravity would perform equal vibrations in equal times, 

 yet the slightest change in the form or weight of the 

 pendulum, the slightest corrosion of any part, or the most 

 minute displacement of the point of suspension, would 

 falsify the results, and there enter many other diffi- 

 cult questions of temperature, resistance, length of vibra- 

 tion, &c. 



Thomson and Tait are of opinion k 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. 

 Although their suggestion is no doubt theoretically cor- 

 rect, 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 



k "The Elements of Natural Philosophy/ part i. p. 119. 



