468 POPULAR LECTURES AND ADDRESSES. 



times the depth of the water in the canal 50 

 times the depth will fulfil this condition the 

 length being always reckoned from crest to crest. 

 Now if the wave-length from crest to crest be 50 

 or more times the depth of the canal, then the 

 velocity of the wave is that acquired by a body 

 falling through a height equal to half the depth 

 of the canal ; if the wave-length be less than that, 

 the velocity can be expressed only by a complex 

 mathematical formula. The results have been 

 calculated ; but I need not put them before you, 

 because we are not going to occupy ourselves 

 with them. 



The conclusion then at which we have arrived is 

 this : supposing at first the velocity of the boat to 

 be such as to make the waves behind it of wave- 

 length short in comparison with the depth of water 

 in the canal : let the boat go a little faster, and 

 give it time until steady waves are formed behind 

 it ; these waves will be of longer wave-length : the 

 greater the speed of the boat, the longer will be the 

 wave-length, until we reach a certain limit ; and as 

 the wave-length begins to be equal to the depth, 



