ON SHIP WA VES. 453 



by him to the Royal Society of Edinburgh. I 

 gave a very general and abstract definition of 

 the term " wave," let us now have it in the 

 concrete : a wave of water produced by a boat 

 dragged along a canal. In one of Scott Russell's 

 pictures illustrating some of his celebrated ex- 

 periments, is shown a boat in the position that 

 he called behind the wave ; and in the rear of 

 the boat is seen a procession of waves. It is 

 this procession of waves that we have to deal 

 with in the first place. We must learn to under- 

 stand the procession of waves in the rear of the 

 canal boat, before we can follow, or take up the 

 elements of, the more complicated pattern which 

 is seen in the wake of a ship travelling through 

 open water at sea. Scott Russell made a fine 

 discovery in the course of those experiments. 

 He found that it is only when the speed of the 

 boat is less than a certain limit that it leaves 

 that procession of waves in its rear. Now 

 the question that I am going to ask is, how 

 is that procession kept in motion ? Does it 

 take power to drag the boat along, and to 



