ON SHIP WAVES. 459 



field to the other, but is the change of colour 

 due to your seeing the sides or lower ends of 

 the ears of corn instead of the tops. A laying 

 down of the stalks is the thing that travels in 

 the wave passing over the corn field. The thing 

 that travels in the w r ave behind the boat is an 

 elevation of the water at the crest and a depres- 

 sion in the hollow. You might make a wave 

 thus. Place over the surface of the water in 

 a canal a wave-form, made from a piece of paste- 

 board or of plastic material such as gutta-percha 

 that you can mould to any given shape; and 

 take care that the water fills up the wave-form 

 everywhere, leaving no bubbles of air in the 

 upper bends. Now you have a constant dis- 

 placement of the water from its level. Now take 

 your gutta-percha form and cause it to move 

 along drag it along the surface of the 

 canal and you will thereby produce a wave. 

 That is one of the best and most convenient of 

 mathematical ways of viewing a wave. Imagine 

 a wave generated in that way ; calculate what 

 kind of motion can be so generated, and you have 



