XI.] THE SEA AND ITS WORK. 171 



wave may be propagated across a broad expanse of water. 

 Drop a stone into a pond, and the same kind of action 

 will be seen : the water all round the spot where the stone 

 falls is first depressed in a little cup, and then rises again, 

 the motion being taken up by the neighbouring water ; and 

 a succession of circles, each wider than the last, spreads 

 over the pond, until the ripples at length die away upon 

 the shore. If any light object, such as a cork, happens 

 to be floating on the surface, it will serve to indicate the 

 the motion of the water below. As the waves reach it, the 

 cork rises and falls, but it is not carried forward by the 

 movement of the water. Exactly the same kind of action 

 may be witnessed at sea. If a gull, for example, is seated on 

 a wave it is simply rocked up and down, and not moved 

 onwards. 



Such simple observations are sufficient to show that the 

 motion of the water is a movement of undulation and not 

 of translation ; it is merely the form of the wave, and not 

 the actual water, that travels. The motion is transmitted 

 from particle to particle, to a great distance ; but the par- 

 ticles themselves perform very small excursions, merely 

 vibrating up and down, or rather revolving in vertical 

 circular paths. The general effect is similar, as has often 

 been pointed out, to that witnessed when a gust of wind 

 sweeps across a field of corn. Notwithstanding the im- 

 pression produced on the observer, he knows that any 

 movement of translation is here quite out of the question ; 

 the stalks are not uprooted and carried across the field, 

 but each stalk simply bends down before the wind and 

 then returns to its erect position. Similarly, in the open sea, 

 the wave, or pulsation, is propagated, but the mass of the 

 water at any given spot remains stationary, except in so far 

 as it vibrates up and down. The mechanical force of the 



