ON SHIP WA VES. 461 



equal all over the surface of the gutta-percha 

 form. Now have done with the gutta-percha. 

 We only had in it imagination. Having imagined 

 it and got what we wanted out of it, discard it. 

 When moving it at exactly this proper speed, you 

 have a free wave. That is a slight sketch of 

 the mode by which we investigate mathemati- 

 cally the velocity of the free wave. It was by 

 observation that Scott Russell found it out ; and 

 then there was a mathematical verification, not 

 of the perfect theoretic kind, but of a kind 

 which showed a wonderful grasp of mind and 

 power of reasoning upon the phenomena that 

 he had observed. 



But still the question occurs to everybody 

 who thinks of these things in an engineering 

 way, how does that procession require work to 

 be done to keep it up ? or does it require work 

 to be done at all ? May it not be that the work 

 required to drag the boat along the canal has 

 nothing to do with the waves after all ? that 

 the formation of the procession of waves once 

 effected leaves nothing more to be desired in 



