ON SHIP WAVES. 457 



cosity comes into play, and causes the results of 

 observation to differ more or less very greatly 

 in some cases, and very slightly in others 

 from what we should calculate on the sup- 

 position of water being a perfect fluid. If 

 water were a perfect fluid, the velocity of pro- 

 gression of a wave in a canal would be smaller 

 the shorter the wave. That of a " long wave " 

 whose length from crest to crest is many 

 times the depth of the canal is equal to the 

 velocity which a body acquires in falling from 

 a height equal to half the depth of the canal. 

 For brevity we might call this height the " speed- 

 height" the height from which a body must 

 fall to acquire a certain speed. The velocity in 

 feet per second is approximately eight times 

 the square root of the height in feet. Examples : 

 a body falls from a height of 16 feet, and it 

 acquires a velocity of 32 feet per second; a 

 body falls from a height of 4 feet, the velo- 

 city is only 16 feet per second; and so on. 

 Thus in a canal 8 feet deep the natural velocity 

 of the "long wave" is 16 feet per second, or 



