ON WAVES. 319 



But the great example of a wave of i\ie first order, is that enormous wave 

 of water which rolls along our shores, bringing the elevation of high tide 

 twice a day to our coasts, our harbours, and inland rivers. This great com- 

 pound wave of the first order is not the less real that its length is so great, 

 that while one end touches Aberdeen, the other reaches to the mouth of the 

 Thames and the coast of Holland. Though the magnitude of this wave renders 

 it impossible for the human eye to take in its form and dimensions at one 

 view, we are able, by stationing numerous observers along different parts of 

 the coasts, to compare its dimensions and to trace its progress at different 

 points, and so to represent its phsenomena to the eye and the mind on a small 

 scale, as to comprehend its form and nature as clearly as we do those of a 

 mountain range, or extensive country which has been mapped on a sheet of 

 paper by the combination together of trigonometrical processes, performed 

 at different places by various observers, and finally brought together and pro- 

 tracted on one sheet of paper. 



As this great wave of the first order is not comprehended by the eye on 

 account of its magnitude, so there is a wave of the fourth order vihich equally 

 escapes detection from that organ, on account of its minuteness. By an un- 

 dulation propagated among the particles of water, so minute as to be altoge- 

 ther insensible to the eye, and only recognised by an organ appropriate to 

 that purpose, there is conveyed from one place to another the wave of sound. 

 This wave, though invisible from its minuteness, is nevertheless of a nature 

 almost identical with the wave of the first order. In air the sound wave is 

 indeed the wave of the first order. It is only in liquids, when the measure of 

 pressure of the fluid mass is different from the measure of the intercorpuscular 

 force, that the phagnomena of the wave of the first order is different from 

 those of the fourth, and that we have one measure for the velocity of the 

 water wave, and another for that of the sound wave. In a gaseous fluid, on 

 the contrary, the measure of the pressure of the mass is also the measure of the 

 intercorpuscular force, and the sound wave becomes identical with the air 

 wave, the fourth order with the first. 



Section I — Wave of the First Order. 

 TJie Wave of Translation, 



Character Solitary. 



o • f Positive. 



" [ Negative. 



Varieties I ^ ', 



\ t orced. 



J , f Wave of Resistance. 



\ Tidal Wave — Sound Wave. 



I believe I shall best introduce this phaenomenon by describing the circum- 

 stances of my own first acquaintance with it. I was observing the motion 

 of a boat wliich was rapidly drawn along a narrow channel by a pair of horses, 

 when the boat suddenly stopped — not so the mass of water in the channel 

 which it had put in motion ; it accumulated round the prow of the vessel in a 

 state of violent agitation, then suddenly leaving it behind, rolled forward with 

 great velocity, assuming the form of a large solitary elevation, a rounded, 

 smooth and well-defined heap of water, which continued its course along the 

 channel apparently without change of form or diminution of speed. I fol- 

 lowed it on horseback, and overtook it still rolling on at a rate of some eight 

 or nine miles an hour, preserving its original figure some thirty feet long and 



