MOTION OF TIDE- WAVE. 



between wave and wave, there likewise they would continually 

 remain, the one wave always keeping before, and the other behind 

 them. 



The experiment may be tried upon a tub of water. Let a 

 pebble drop into the centre of it. Rings of waves will imme- 

 diately be formed round the place where it falls, and they will 

 appear to move outwards from the place of the fall towards the 

 edge of the tub. If a cork be placed anywhere upon the water, 

 it will not be carried by these waves towards the edge of the 

 tub, but will float in the same place, the waves passing succes- 

 sively under it, and the cork rising and sinking as the crest and 

 hollow pass it. 



If we observe the waves of the sea breaking on a level strand, 

 we shall soon be convinced that their apparent progressive motion 

 does not affect the water, for if it did, the sea would soon now 

 in upon the shores, and inundate the adjacent country. So 

 far, however, from the water's partaking of the apparent motion 

 of the waves in approaching the shore, this motion of the waves 

 continues the same even when the water is retiring. If we 

 observe a flat strand when the tide is ebbing, we shall still find 

 the waves moving towards the shore. 



16. That this apparent motion of water in a state of undula- 

 tion is a mere optical illusion we cannot therefore doubt. But 

 we are naturally curious to learn what is the cause of this 

 illusion. That a progressive motion takes place in something, we 

 have proof from the evidence of our eyes. That no progressive 

 motion takes place in the liquid we have also proof, from the 

 evidence of our eyes, and from other still more unquestionable 

 testimony. To what then does. this progressive motion belong? 

 we answer, to the form of the wave, and not to the liquid that 

 composes it. 



To make this apparent, let A B c D E, &c. (fig. 4), represent 

 the surface of the sea, c and L being the crests of two suc- 

 cessive waves, and <J the hollow between them, and let x Y 

 represent the bottom of the sea. After a given interval, ten 

 seconds for example, let the position of the waves be a b c d e, &c., 

 the motion being directed from A. towards R. Now this motion 

 of the waves is produced in the following manner : The water 

 which was at A sinks, during the interval of ten seconds, to a, 

 the water which was at B sinks to b, that which was at c to c, 

 that which was at D rises to d, that which was at E rises to e, 

 that which was at F to /, and so on. Thus, in the interval, all 

 parts of the water on one side of a certain point sink, and all 

 those at the other side rise, the extent to which they rise and 

 sink being such, that the surface assumes the new position 



141 



