466 POPULAR LECTURES AND ADDRESSES. 



the canal supposing that the water is free from 

 viscosity is just equal to the work required 

 to generate the procession of waves lengthening 

 backwards behind the boat at half the speed of 

 the boat. The rear of the procession travels for- 

 wards at half the speed of the boat ; the proces- 

 sion lengthens backwards relatively to the boat at 

 half the speed of the boat. There is the whole 

 thing ; and if you only know how to calculate 

 the energy of a procession of waves, assuming 

 the water free from viscosity, you can calculate 

 the work which must be done to keep a canal 

 boat in motion. 



But now note this wonderful result : if the 



motion of the canal boat be more rapid than 



the most rapid possible wave in the canal 



(that is, the long wave), it cannot leave behind 



it a procession of waves it cannot make waves, 



properly so called, at all ; it can only make a 



hump or a hillock travelling with the boat, as 



shown in another of Scott Russell's drawings. 



What would you say of the work required to 



move the boat in that case ? You may answer 



