268 On ñe Tramlation of Waves of Water, [No. 3, 



not permanently affected by the passage of the wave, but oscillates 

 between two fixed points upon flood and ebb, according to wave laws 

 which will presently be indicated. 



7. The progresa of a wave then may be described as the transla- 

 tion of a shape or form, in which the partióles are continuously 

 changing — but these partióles although they are successively cast oíf, 

 have a certain motion communicated to them by the wave, though 

 it is not that of the wave itself. 



8. The sea side observer would with attentive watching perceive, 

 that the piece of driffc wood or foam is actually carried forward by 

 the crest of the wave to a certain extent, though not in anything 

 like the ratio of progression of the wave itself, and that when the 

 wave has passed, it is carried baekward in the succeeding hollow, so 

 that it always occupies the same mean position ; and in like manner, 

 a boat or a ship, and the termination of the salt water, are carried a 

 certain distance up a river by the flood or crest of a tidal wave, and 

 down again by the ebb or hollow, so that if uninfluenced by other 

 causes they will recover their original position. 



9. It has been mathematically demonstrated, and direct experi- 

 ment has established, that the particles of water of which a wave is 

 composedj actually move in a circle ; or an ellipse ; the formation 

 of which varíes in proportion to the mass of the wave, and the depth 

 of the water. 



10. When the wave is small, and the water deep, the particles 

 move nearly, if not quite, in a perfect circle, — in other words the 

 vertical and horizontal displacements are about equal ; but when the 

 wave is very large, as the tidal wave, and the water shallow, the 

 vertical displacement is wholly insignificant to the horizontal, and 

 the motion of the particle, measured from any fixed point, is an 

 extremely fíat ellipse, of which the horizontal is the major axis. 



11. In the accompanying sketch, a wave is supposed to be 

 travelling along a level sheet of water from X. to Z. — A. is the centre 

 of the preceding hollow i — B. the middle of the anterior slope : — 

 C. the crest of the wave : — D. the middle of the posterior slope : — 

 and E. the centre of the succeeding hollow. A particle of water 

 which is at A. will be carried baekward or towards the wave : — At 

 B. its horizontal motion will be neutralized and it will be found 



