ON SHIP WA VES. 465 



In a canal especially viscosity is very effective, 

 because the water has to flow more or less across 

 the bottom and up and down by the banks ; 

 so that we have not there nearly the same freedom 

 that we have at sea from the effects of viscosity 

 in respect to waves. The rear of the procession 

 travels forward at half the speed of the ship, 

 if the water be very deep. What do I mean 

 by very deep ? I mean a depth equal to 

 at least one wave-length ; but it will be nearly 

 the same for the waves if the depth be three- 

 quarters of a wave-length. For my present pur- 

 pose in which I am not giving results with 

 minute accuracy, we will call very deep any depth 

 more than three-quarters of the wave-length. 

 For instance, if the depth of the water in the 

 canal is anything more than three-quarters of 

 the length from crest to crest of the waves, the 

 rate of progression of the rear of the procession 

 will be half the speed of the boat. Here then 

 is the state of the case. The boat is followed by 

 an ever-lengthening procession of waves; and 

 the work required to drag the boat along in 

 VOL. III. II H 



