Tracks of Animals in the Forest Marble. 541 



which waves of given heights, under similar circumstances of 

 wind and current, begin to swell and conform to the direction 

 of the shore, would afford reasonable data for determining the 

 depths to which the undulatory movement is propagated ; and 

 I throw out this as a hint to such persons as have the requisite 

 opportunities for making such observations, and are interested 

 in solving this question. 



A wave is clearly a parcel of water heaped up by the con- 

 cussion between any external impulse and the resistance of its 

 own inertia. If the impulse is single, as when a stone is thrown 

 into still water, the disturbance subsides through a succession 

 of oscillations, like those of a pendulum, till the equilibrium is 

 restored. The wave formed in the parcel of water first affected, 

 as it descends, communicates the impulse laterally to the next 

 parcel, which consequently rises, and falling again transmits 

 the impulse to a third, and so on, until the original impulse is 

 lost by expansion. If the impulse is continued, as by the pro- 

 longed action of wind on the surface of water, the waves 

 maintain their full force, or rather increase gradually, so long 

 as the fresh impulse received during each oscillation is greater 

 than that lost by lateral or vertical dispersion. Hence, when 

 a wind begins to act on a calm surface of water, the waves, at 

 first small, gradually wax higher and broader, and no doubt 

 progress downwards in the same ratio ; and what sailors call 

 ' a swell ' gets up, after a wind has blown on the sea for a cer- 

 tain time. This swell continues, owing to the vast momentum 

 acquired by the agitated waters, long after the wind which pro- 

 duced it has gone down or shifted, and gradually subsides as it 

 gradually commenced. It is often vulgarly supposed that 

 there is a real movement of the water in the direction of the 

 waves, and indeed the eye has some difficulty in detecting 

 that this is not the case. On the contrary, waves caused by 

 wind frequently move with great rapidity in the opposite di- 

 rection to that in which the body of the water is carried by 

 tides or currents. 



The long axes of waves are of course transverse to the im- 

 pelling force. In the annexed diagram, each of the waves alter- 

 nately rises and falls. 



