ON SHIP WAVES. 469 



or twice the depth, or three times the depth, we 

 approach a wonderful and critical condition of 

 affairs we approach the case of constant wave 

 velocity. There will still be a procession of waves 

 behind the boat, but it will be a shorter procession 

 and of higher waves ; and this procession will not 

 now lengthen astern at half the speed of the boat, 

 but will lengthen perhaps at a third, or a fourth, or 

 perhaps at a tenth of the .speed of the boat. We 

 are approaching the critical condition : the rear of 

 the procession of waves is going forward nearly as 

 fast as the boat. This looks as if we were coming 

 to a diminished resistance ; but it is not really so. 

 Though the procession is lengthening less rapidly 

 relatively to the boat than when the speed was 

 smaller, the waves are very much higher ; and we 

 approach almost in a tumultuous manner to a 

 certain critical velocity. I will read you presently 

 Scott Russell's words on the subject. Once that 

 crisis has been reached, away the boat goes merrily, 

 leaving no wave behind it, and experiencing no 

 resistance whatever if the water be free from 

 viscosity, but in reality experiencing a very large 



