94 WAVES OF THE SEA 



Some interesting deductions can be made from 

 the above record. 



The mean between the period at commencement 

 ( 1 7 seconds ) and that next day ( 1 3 seconds ) is 

 15 seconds ; and in 24 hours 5,760 waves of this 

 period would have discharged themselves upon the 

 shore . The length of a i 5 -second wave is 1,153 

 feet, so that 5,760 such waves in series would 

 occupy a space of 1,090 geographical miles. The 

 length of the waves at the front of the group was 

 1,481 feet, and of those at the rear 866 feet. If 

 we suppose the waves travelling freely after the 

 storm, the rate of progress of the group, if reckoned 

 by that of a 15 -second wave, would be, not 45, 

 but 22 J knots, 1 so that the advance per 24 hours 

 would be 540 geographical miles, and the interval 

 between the storms and the arrival of the swell 

 was " some days." 



The character of the group of waves at the 

 moment when the storm ceased must have been 



1 Vide Nature, vol. xvi., 1877, p. 343, for Osborne Reynolds on 

 the relation between group-velocity and wave-velocity in deep 

 water. In deep water a group of trochoidal waves travelling 

 freely under the action of gravity advances at half the speed 

 of the individual waves. If we follow the motion of the first 

 wave of the group, we shall find that it dies out, and the wave 

 behind it has now taken the lead. If, on the other hand, we 

 watch the last wave of the group, we shall soon find that 

 another one has appeared behind it, and the sum total of these 



