LORD KELVIN 97 



In 1893 Lord Kelvin gave a discourse on 'Isoperi- 

 metric Problems ' at the Royal Institution, in which he 

 attempted to describe the nature of this general problem ; 

 it is that technically called ' Determining a' minimum ' ; 

 and he began with the task which faced Dido of old to 

 surround the most valuable piece of land with a cowhide, 

 i.e. to draw the shortest possible line around it. A 

 similar problem is, to build a railway-line through 

 undulating country at the smallest possible cost ; and one 

 very different in appearance, but related to those already 

 cited, owing to Lord Kelvin's consummate power of dis- 

 covering analogies between phenomena apparently uncon- 

 nected, is the condition of stability of water rotating in 

 an ellipsoidal vessel, and a number of similar problems. 

 Kelvin's work on Elasticity is no less far-reaching ; in 

 Karl Pearson's great treatise on that subject, no less 

 than one hundred pages are filled with Kelvin's con- 

 tributions. 



Lord Kelvin was also the author of a theory of the 

 nature of the ultimate particles of matter the atoms, 

 lie imagined them to consist of 'vortex rings in the 

 ether,' the ether being conceived as a frictionless fluid, all- 

 present, even filling the interstices between the atoms, or 

 ultimate particles of matter. Yortex rings in air, some- 

 times made by smokers, are elastic ; they cannot be cut 

 without being destroyed ; and, in a frictionless fluid, their 

 rotatory motion would be eternal, if once impressed. 

 Recent discoveries may lead to the modification of this 

 theory of the nature of matter ; but it has much in its 

 favour. 



Kelvin was a strong partisan of Joule's work on the 

 equivalence of heat and work. It was believed up to 

 1850 that the heat developed on compressing a gas was 

 ' caloric,' squeezed out of the gas, as one might squeeze 

 water out of a sponge ; but Kelvin taught that heat must 



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