86 WAVES OF THE SEA 



change of speed causes the wave -crests to close 

 up, so that the space separating them gets less and 

 less as they approach the shore, but the interval 

 of time between the arrival there of successive 

 crests is unchanged. Consequently, by timing the 

 arrival of a number of breakers, or the passage 

 of a fixed point by a number of waves just before 

 breaking, we know at once the period of these 

 waves, not only as we see them in shallow water, 

 but as they were in water so deep that the wave dis- 

 turbance did not reach nearly to the bottom. From 

 the period we can calculate the speed in deep water 

 by the mathematical theory of waves, using the 

 formula : 



Period (in seconds) = speed of wave in feet per second -7- 5^ 

 (nearly), or, more roughly: 



Period (in seconds) = speed of wave in knots per hour -:- 3 ; 



and we can calculate the wave-length for deep 

 water thus : 



Square of period = length of wave -i- 5^. 



The breakers which arrive somewhat irregularly 

 during storms do so at intervals which, as far as 

 I have noticed them, do not differ much from the 

 intervals observed on board ship in the deep sea 

 during storms ; but after storms the period of the 



