ON SHIP WAVES. 467 



that question at once : it would require no work ; 

 start it, and it will go on for ever. Every one 

 understands that a curling stone projected along 

 the ice would go on for ever, were it not for the 

 friction of the ice ; and therefore it must not 

 seem so wonderful that a boat started moving 

 through water would also go on for ever, if the 

 water were perfectly fluid : it woiild not> if it is 

 forming an ever-lengthening procession of waves 

 behind it ; it would go on for ever, if it is not 

 forming a procession of waves behind it. The 

 answer then simply is, give the boat a velocity 

 greater than the velocity of propagation of the 

 most rapid wave (the long wave) that the canal 

 can have ; and in these circumstances, ideal 

 so far as nullity of viscosity is concerned, it 

 will travel along and continue moving without 

 any work being done upon it. I have said that 

 the velocity of the long wave in a canal is equal 

 to the velocity which a body acquires in falling 

 from a height equal to half the depth of the 

 canal. The term " long wave " I may now further 

 explain as meaning a wave whose length is many 



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