430 SCIENCE PROGRESS 



luminiferous ether disturbed by ordinary matter ? With a 

 constant velocity of propagation, as in pure ether, each group 

 remains unchanged. But how about the propagation of light 

 sequences in a transparent medium like glass ? " References 

 are also made to difficulties that might arise in connection with 

 refraction or interference phenomena if these were dealt with by 

 consideration of groups of waves. Then again, taking the 

 production of light from a molecule as a sudden beginning of a 

 long regular group of waves followed by a gradual falling off, we 

 are confronted with the question, how would irregularity invade 

 the regular group in its passage through a dispersive medium ? 



With the single exception of the papers dealing with Ship 

 Waves, in which Lord Kelvin had a special interest arising from 

 his earlier hydrodynamical work, the problems solved in the 

 later Waves Papers are the water wave analogues of the Optical 

 problems referred to in the above quotations. We have " On the 

 Front and Rear of a Free Procession of Waves in Deep Water" 

 appearing in 1887 and later in 1904. In his last paper we have 

 the graphical solutions for the motions of a finite group of 

 waves, and the effect of a sudden beginning of regular vibra- 

 tions is represented in the problem of determining the effect 

 of a suddenly applied periodically varying pressure acting at 

 a certain region of the water surface. In this connection it 

 may be of interest to note that the optical analogue of the 

 Ship Waves problem is the problem of the passage of a plane 

 light pulse from air into glass or other dispersive medium, 

 and that the Ship Waves solutions have since been used 

 to illustrate the modus-operandi of the prism. It is clear 

 that the intention of these papers is to provide illustrations 

 of wave motion which might provide definite information as 

 to the process of dispersion, and which might be useful in 

 helping to clear up some of the difficulties .which still re- 

 mained in connection with the theory of group-velocity and 

 the propagation of waves in dispersive media. They were, 

 so to speak, models illustrating the Optical problems referred 

 to in the Baltimore Lectures and in his papers on Atoms and 

 Electrons. 



From this point of view the most important paper of the 

 section is that entitled " On the Waves produced by a Single 

 Impulse in Water of any Depth, or in a Dispersive Medium " 

 {Phil. Mag., 1887). In this paper the displacement pro- 



