PREFACE. XV 



" It is desirable that an attempt should also be made to determine on which of 

 the above hypotheses the appearances both of the bright rings and the recently 

 discovered dark ring may be most satisfactorily explained; and to indicate any causes 

 to which a change of form such as is supposed from a comparison of modern with the 

 earlier observations to have taken place, may be attributed." 



It is sufficient to mention here that Maxwell bestowed an immense amount of labour 

 in working out the theory as proposed, and that he arrived at the conclusion that "the 

 only system of rings which can exist is one composed of an indefinite number of unconnected 

 particles revolving round the planet with different velocities according to their respective 

 distances. These particles may be arranged in a series of narrow rings, or they may move 

 about through each other irregularly. In the first case the destruction of the system will be 

 very slow, in the second case it will be more rapid, but there may be a tendency towards 

 an arrangement in narrow rings which may retard the process." 



Part of the work, dealing with the oscillatory waves set up in a ring of satellites, 

 was illustrated by an ingenious mechanical contrivance which was greatly admired when 

 exhibited before the Royal Society of Edinburgh. 



This essay, besides securing the prize, obtained for its author great credit among 

 scientific men. It was characterized by Sir George Airy as one of the most remarkable 

 applications of Mathematics to Physics that he had ever seen. 



The suggestion has been made that it was the irregular motions of the particles which 

 compose the Rings of Saturn resulting on the whole in apparent regularity and uni- 

 formity, which led Maxwell to the investigation of the Kinetic Theory of Gases, his first 

 contribution to which was read to the British Association in 1859. This is not unlikely, 

 but it must also be borne in mind that Bernoulli's Theory had recently been revived by 

 Herapath, Joule and Clausius whose writings may have drawn Maxwell's attention to the 

 subject. 



In 1860 King's College and Marischal College were joined together as one institution, 

 now known as the University of Aberdeen. The new chair of Natural Philosophy thus 

 created was filled up by the appointment of David Thomson, formerly Professor at King's 

 College and Maxwell's senior. Professor Thomson, though not comparable to Maxwell as a 

 physicist, was nevertheless a remarkable man. He was distinguished by singular force of 

 character and great administrative faculty and he had been prominent in bringing about 

 the fusion of the Colleges. He was also an admirable lecturer and teacher and had done 

 much to raise the standard of scientific education in the north of Scotland. Thus the choice 

 made by the Commissioners, though almost inevitable, had the effect of making it appear 

 that Maxwell failed as a teacher. There seems however to be no evidence to support such 

 an inference. On the contrary, if we may judge from the number of voluntary students 

 attending his classes in his last College session, he would seem to have been as popular as a 

 professor as he was personally estimable. 



