ON SHIP WA VES. 463 



of progress of a wave is one thing ; the velocity 

 of the front of a procession of waves, and of 

 the rear of a procession of waves, is another 

 thing. Stokes made a grand new opening, show- 

 ing us a vista previously unthought of in dynam- 

 ical science. As was his manner, he did it 

 merely in an examination question set for the 

 candidates for the Smith prize in the University 

 of Cambridge. I do not remember the year, 

 and I do not know whether any particular candi- 

 date answered the question ; but this I know, 

 that about two years after the question was 

 put, Osborne Reynolds answered it with very 

 good effect indeed. In a contribution to the 

 Plymouth Meeting of the British Association 

 in 1877 (see Nature 23 Aug. 1877, pages 

 343 4), in which he worked out one great 

 branch at all events of the theory thus pointed 

 out by Stokes, Reynolds gave this doctrine of 

 energy that I am going to try to explain ; and 

 a few years later Lord Rayleigh took it up and 

 generalised it in the most admirable manner, 

 laying the foundation not only of one part, but 



