72 



ON THE PHYSIOLOGICAL CAUSES OF 



volving in the circles a and c to be simultaneously in their lowest 

 positions. 



The respective particles of water revolve in these circles in 

 the direction marked by the arrows. The dotted curves repre- 

 sent other positions of the passing waves, at equal intervals of 

 time, partly before the assumption of the ABC position (as for 

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

 crests between b and c). The positions of the crests are marked 

 with figures. The same figures in the three circles, show where 

 the respective revolving particle would be, at the moment the 

 wave assumed the corresponding form. It will be noticed that 

 the particles advance by equal arcs of the circles, as the crest of 

 the wave advances by equal distances parallel to the water level. 



In the circle b it will be further seen, that the particle oi 

 water in its positions 1, 2, 3, hastens to meet the approaching 



wave-crests, 1, 2, 3, rises on its left hand side, is then carried on 

 by the crest from 4 to 7 in the direction of its advance, after- 

 wards halts behind it, sinks down again on the right side, and 

 finally reaches its original position at 13. (In the Lecture itself, 

 Fig. 2 was replaced by a working model, in which the movable 

 particles, connected by threads, really revolved in circles, while 

 connecting elastic threads represented the surface of the water.) 



All particles at the surface of the water, as you see by this 

 drawing, describe equal circles. The particles of water at dif- 

 ferent depths move in the same way, but as the depths increase, 

 the diameters of their circles of revolution rapidly diminish. 



In this way, then, arises the appearance of a progressive motion 

 along the surface of the water, while in reality the moving par- 

 ticles of water do not advance with the wave, but perpetually 

 revolve in their small circular orbits. 



