112 LECTURES TO SCIENCE TEACHERS. 



I have just spoken of, namely surface-friction and eddy- 

 making resistance. The stream-line motions being the 

 same in character as those we have been considering, we 

 shall still have at each end an excess of pressure, and 

 along the sides a defect of pressure, which will tend the 

 one to force up the sheet of ice and the other to suck it 

 down. If now we remove the ice, the water will obviously 

 rise in level at each end, in order that excess of hydrostatic 

 head may afford the necessary reaction against the excess 

 of pressure, and the water will sink by the sides, in order 

 that defect of hydrostatic head may afford reaction against 

 the defect of pressure. 



The hills and valleys which thus commence to be formed 

 in the water are, in a sense, waves, and though originating 

 in the stream-line forces of the body, yet when originated, 

 they come under the dominion of the ordinary laws of 

 wave-motion, and to a large extent behave as independent 

 waves ; and in virtue of their independent action they 

 modify the stream-line forces which originated them, and 

 alter the pressures which are acting upon the surface of 

 the ship. 



The exact nature of this alteration of pressure, in any 

 given case, we have no means of predicting ; but we can 

 be quite sure it must operate to alter the balance of for- 

 ward and backward forces in such a way as to cause re- 

 sistance ; for we see that the final upshot of all the 

 different actions which take place is this that the ship in 

 its passage along the surface of the water has to be con- 

 tinually supplying the waste of an attendant system of 

 waves, which, from the nature of their constitution as in- 

 dependent waves, are continually diffusing and transmitting 

 themselves into the surrounding water, or, where they 

 form what is called broken water, crumbling away into 

 froth. Now, waves represent energy, or work done, and 

 therefore all the energy represented by the waves wasted 

 from the system attending the ship is so much work done 

 by the propellers or tow ropes which are urging the ship. 

 So much wave-energy wasted per mile of travel is so much 

 work done per mile, and so much work done per mile is so 

 much resistance. 



The surface of the water thus admits of an escape, as 

 it were, of the pressures which arise from the inertia of 



