96 WAVES OF THE SEA 



as the period decreased, and observations were dis- 

 continued when the period was 13 seconds. Theory 

 shows that when deep-sea waves are left to them- 

 selves, the time which they take to flatten out so 

 as to become invisible is proportional to the square 

 of the wave-length. 1 Hence the longer waves are 

 more permanent. Again, waves of all lengths are 

 reduced to the same small speed by the time they 

 break upon the shore ; so that waves originally long 

 and swift are shut up, or telescoped, to a much 

 greater extent than the slower ones, and they 

 undergo in the process a correspondingly greater 

 increase in height. On both accounts, therefore, 

 the height of the breakers given by the swifter 

 waves is increased out of all proportion to their 

 original height during the storm. 



The Height of the Swell at Sea during Storms 



I shall now attempt to arrive at some conclu- 

 sion as to the height during storms of the waves 

 longer than those then measured, or that part of 

 the wave-disturbance which is termed then, and 

 afterwards, " the swell.'* On board ship during 

 storms the swell is felt in the movement of the 

 vessel more distinctly than it is seen, but it often 

 can be perceived as a sort of broad band, lighter 

 1 Vide J. Boussinesq, Comptes Rendus, cxxi. 



