SOME VIEWS ON LORD KELVIN'S WORK 429 



tension on Water Waves illustrated by experiments carried out 

 with the assistance of Prof. Helmholtz and Prof. James Thom- 

 son on one of his yachting expeditions, when becalmed in the 

 Sound of Mull. The series of " Stationary Waves in Flowing 

 Water " is evidently undertaken with the view of leading up to 

 the problems of Ship Waves and Waves due to Wind. The solu- 

 tion of the Ship Waves problem was obtained long before it was 

 published (in 1906). In a manner the investigation of water 

 waves was more or less a recreation study to Lord Kelvin, being 

 a natural interest aside from his more pressing practical affairs 

 and from the deeper problems of matter and ether, and yet 

 bearing on both and providing scope for the applications of 

 his skill in Fourier mathematics, of which these papers contain 

 many examples. 



But from 1884 onwards the main purpose of the continued 

 series of papers on Water Waves ceased to be merely the 

 hydrodynamical value of the solutions of the several problems 

 with which they are concerned, though these are interesting 

 enough in themselves, and though Lord Kelvin preferred to 

 confine himself in the main to their strictly hydrodynamical 

 bearing as continuations of the Stationary Waves Group. The 

 main interest of the later hydrodynamical papers is to be found 

 in their bearing upon Optical questions requiring elucidation, as 

 is clearly indicated in Lectures V to X of the Baltimore Lectures. 

 "Take any conceivable supposition as to the origin of light, in a 

 flame, or a wire made incandescent by an electric current, or any 

 other source of light. One molecule, of enormous mass in 

 comparison with the luminiferous ether that it displaces, gets a 

 shock, and it performs a set of vibrations until it comes to rest, 

 or gets a shock in some other direction. . . . We thus see that 

 light is essentially composed of groups of waves ; and if the 

 velocity of the front or rear of a group of waves, or of the centre 

 of gravity of a group, differs from the wave velocity of absolutely 

 continuous sequences of waves, in water or glass, or other 

 dispersively refracting mediums, we have some of the ground 

 cut from under us in respect to the velocity of waves of light in 

 all such mediums. I mean to say, that all light consists of groups 

 following one another irregularly, and that there is a difficulty to 

 see what to make of the beginning and end of the vibrations of a 

 group." In Lecture VIII we find later: "A question is now 

 forced upon us, — What is the velocity of a group of waves in the 



