276 Formation of a Wave [No. 3, 



tion of transfer ; this effect being produced by the plug forcing 

 onwards into the canal the water it displaces. 



2. If the plug remains motionless after it has reached the bottom, 

 and the half-wave it has forced up is left to itself, the following 

 process will take place. The higher parts of the half-wave will sink 

 by their own weight and press up its less elevated parts ; and these 

 in their turn will by their weight press up the surface of the hitherto 

 still water of the canal beyond the originally formed half-wave. By 

 this process the half-wave L E which was generated by the plug 

 will form itself into a whole-wave of less height and greater length 

 than the half-wave, like Gr K in fig. 3. This whole-wave will move 

 freely along the canal, elevating the surface of the water at each 

 place as it passes it, and then depressing the surface again to the 

 original level. The slope of the back of this wave will, in general, 

 be longer than the forepart of the wave, because this slope is formed 

 by the sinking of the elevated water merely by its weight ; whereas 

 the forepart of the wave is formed (as above described) by the forced 

 action of the plug, and this force is supposed to be much greater 

 than the mere difference of weight arising from the different eleva- 

 tions of the different parts of the wave. This free whole-wave is 

 represented in fig. 3. The volume of water in this whole-wave, 

 which moves solitarily and freely along the canal, is the same as the 

 volume of water in the forced half-wave from which it grew, and 

 therefore is equal to the volume of water displaced by the plug. 



3. The length of the generated half- wave, (and therefore also the 

 length of the free whole-wave which finally moves along the canal,) 

 depends upon the rapidity with which pressure is communicated 

 through water. This rapidity depends upon the exciting cause. A 

 very extreme example of the communication of pressure through 

 water is seen in the velocity of sound through water, which has been 

 found by careful experiments in the Lake of Geneva to be about 

 eight-ninths of a mile in one second, or 3200 miles an hour. At this 

 rate is the pressure communicated, which causes the minute but 

 rapid vibrations of the water which produce the sound. Another 

 example is the velocity of the tidal-wave up the Hooghly, which 

 moves (as Mr. Obbard states) at 24 miles an hour. I have myself 

 made experiments on the great swell-waves at the Equator and found 



