1893. BIOLOGICAL THEORIES. 423 



over the side of the boat are usually not in the best possible condition 

 for making scientific observations. The end of a pier has, however, 

 some advantages over a boat under way ; but in a boat at anchor is 

 the place where the observations can best be made. The best time 

 for the purpose is when the waves are large, and especially when 

 their crests are sharp-edged and not rounded, and when there is no 

 wind and no breakers or " white horses." Most of the points, how- 

 ever, may be made out from the pier-head, though in that case the 

 path of the particle or of the mass is not a circle, as in deep water, 

 but an ellipse with the long axis horizontal. 



A small floating body almost or completely submerged is not 

 carried along with the waves, but moves when on the crest of a wave 

 more slowly than the wave, but in the same direction. So soon as it 

 has fallen behind the crest into the trough it moves in the opposite 

 direction as if, having slipped off one crest, it were hurrying to climb 

 up to the summit of the next one. In the open sea the extent of this 

 to-and-fro movement is equal to that of the up-and-down movement, 

 so that the body — provided it be very small — constantly travels in a 

 circular orbit, the diameter of which is equal to the height of the 

 crests above the troughs. If there be no wind it will hardly progress 

 at all, but will return always to almost the same place. The very 

 slight progress of particles near the surface may be neglected, and 

 the path of each particle may be regarded as a circle. 



Let us now suppose the water divided into a series of layers, 

 which, if the water were at rest, would each have a uniform thickness 

 of, say, one inch. The passage of waves would not cause the water 

 of one layer to mix with that of others ; but since the water of the 

 superficial layer in a trough is moving in the opposite direction to 

 that in the crest behind it, it follows that the length of the portion 

 between these two points, measured in the direction in which the 

 waves are travelling, is getting less, and this portion of the layer 

 must therefore be getting thicker, while the portion on the back of 

 the wave is getting thinner. The total amount of the elevation of 

 the crest is the net result, the sum, of the thickenings of all the 

 successive layers of water, from the surface down to the bottom. 

 The depression in the trough is similarly the net result of the thinning 

 of all the layers below this region. 



Another mode of observation is even more instructive. A small 

 loose tangle of zoophytes, or of fine seaweed, or a very loose ball of tow, 

 thrown into the water, is seen to rise and fall and to move to and fro ; 

 but though each part of it describes a circle, the mass does not rotate, 

 nor yet do its parts move all together. One part of the mass is at 

 the top of its circular orbit before another, and hence the mass 

 changes in form. The changes of form, or deformations, may be 

 most easily described if we suppose the mass to be such that in still 

 water it would be spherical, that is, a sphere of water rendered visible 

 by the suspension within it of loose tow or seaweed, or, better still. 



