464 POPULAR LECTURES AND ADDRESSES. 



of the whole, of the theory of the velocity of 

 groups of waves. 



The theory of the velocity of groups of waves, 

 on which is founded the explanation of the 

 wave-making resistance to ships whether in a 

 canal or at sea, I think I have explained in 

 such a way that I hope every one will 

 understand the doctrine in respect to waves in 

 a canal ; it is more complex in respect to waves 

 at sea. I shall try to give you something on 

 that part of the subject ; but as to the dynami- 

 cal theory, you will see it clearly in regard to 

 waves in a canal. If Scott Russell's drawing were 

 continued backwards far enough, it would show 

 an end to the procession of waves in the rear 

 of the boat ; and the distance of that end would 

 depend on the time the boat had been travelling. 

 You will remember that we have hitherto been 

 supposing water to be free from viscosity; but 

 in reality water has enough of viscosity to 

 cause the cessation of the wave procession at 

 a distance corresponding to 50 or 60 or 100 

 or 1000 wave-lengths in the rear of the ship. 



