62 ON THE PHYSIOLOGICAL CAUSES OF 



the form of waves, which spread in rings over the surface of 

 the water. The circles of waves continue to increase even after 

 rest has been restored at the point first affected. At the same 

 time the waves become continually lower, the further they are 

 removed from the centre of motion, and gradually disappear. On 

 each wave- ring we distinguish ridges or crests, and hollows or 

 troughs. 



Crest and trough together form a wave, and we measure its 

 length from one crest to the next. 



While the wave passes over the surface of the fluid, the 

 particles of the water which form it do not move on with it. 

 This is easily seen, by floating a chip of straw on the water. 

 When the waves reach the chip, they raise or depress it, but 

 when they have passed over it the position of the chip is not 

 perceptibly changed. 



Now a light floating chip has no motion different from that 

 of the adjacent particles of water. Hence we conclude that 

 these particles do not follow the wave, but, after some pitching 

 up and down, remain in their original position. That which 

 really advances as a wave is, consequently, not the particles of 

 water themselves, but only a superficial form, which continues 

 to be built up by fresh particles of water. The paths of the 

 separate particles of water are more nearly vertical circles, in 

 which they revolve with a tolerably uniform velocity, as long 

 as the waves pass over them. 



In Fig. 2 the dark wave-line ABC represents a section of the 

 surface of the water over which waves are running in the direction 

 of the arrows above a and c. The three circles a, b, and c represent 

 the paths of particular particles of water at the surface of the wave. 

 The particle which revolves in the circle b is supposed, at the time 

 that the surface of the water presents the form A B C, to be at its 

 highest point B, and the particles revolving in the circles a and 

 c to he simultaneously in their lowest positions. 



The respective particles of water revolve in these circles in the 

 direction marked by the arrows. The dotted curves represent other 

 positions of the passing waves, at equal intervals of time, partly 

 before the assumption of the ABC position (as for the crests be- 

 tween a and b), and partly after the same (for the crests between b 



