O.V SHIP WAVES. 487 



result from the law that the group-velocity is half 

 the wave-velocity the velocity of a group of 

 waves at sea is half the velocity of the individual 

 waves. Follow the crest of a wave, and you see 

 the wave travelling through the group, if it forms 

 one of a group or procession of waves. Look, 

 quite independently of the ship, at a vast pro- 

 cession of waves, or imagine say fifty waves ; look 

 at one of those waves, follow its crest ; in imagina- 

 tion fly as a bird over it, keeping above the crest 

 as a bird in soaring does sometimes, and, begin- 

 ning over the rear of the procession, a hundred 

 yards on either side of the ship's wake, you will 

 find the waves get larger and larger as you go 

 forwards. Then go backwards through the pro- 

 cession, and you will see the waves get smaller 

 and smaller and finally disappear. You have now 

 gone back to the rear of the procession ; a small 

 wave increases and travels uniformly forward, and, 

 while the crest of each wave always goes on with 

 the velocity corresponding to the length of the 

 wave, the rear of the procession travels forward 

 at half the speed of the wave : so that every wave 



