26 WAVE DIMENSIONS 



common case) would be about P/-* nautical miles from front to rear. 

 If the waves averaged, say, 800 feet from crest to crest at the time the 

 squall first developed, it would act on only about 10 of them; it would 

 influence a proportionately greater number if it developed earlier in 

 the gale while the waves were shorter, or a smaller number if it devel- 

 oped after a really long sea was already running. And squalls of 

 wider extent would act upon a correspondingly larger number of 

 waves. A case of this sort is on record for the south coast of England, 

 when a train of 139 large breakers was observed, the product of a single 

 violent squall, with periods so long (average 19 seconds) that the 

 group as a whole must have extended over a distance of 49 miles while 

 they were still out in deep water. This group occupied three-quarters 

 of an hour and it was preceded by five groups, each of four to seven 

 still larger breakers, with average periods of 20 seconds. These 

 groups, occupying one to two minutes had, no doubt, been engendered 

 by a series of 1- to 2-minute gusts, and had outrun the more extensive 

 group produced by the three-quarters of an hour squall. 6 



But the still fiercer gusts, lasting only a few seconds, which, in turn, 

 punctuate every squall, extend over such short distances that they 

 affect only part of one of the individual waves, if the latter have/ 

 advanced beyond the very earliest stages in their development. Conse- 

 quently, the sizes of the largest waves produced by a squall correspond 

 to the average velocity of the wind within the latter, not to the very 

 highest velocities to which the wind may rise momentarily. 



When we remember that individual squalls travelling at rates of 

 20 to 40 miles per hour have been shown by self-registering instru- 

 ments at meteorological stations to have advanced unbroken for dis- 

 tances up to 1,000 miles or more, there is nothing astonishing in the 

 well-established fact that the trains of very large waves that they 

 produce may do the same. 



THE LENGTHS OF WAVES 



Anyone who has seen ripples grow 7 to whitecaps under a rising 

 wind and who has watched whitecaps develop into a sea knows that 

 the waves grow longer as they gain in height. And the linear distance 

 from crest to crest increases much more rapidly than does the absolute 

 height of the waves, provided the shape of the latter (i. e., the ratio 

 between its length and its height) continues approximately the same, 

 for waves are invariably many times as long as they are high. If 

 a 5-foot wave, 100 feet long (a common proportion of height to 

 length), doubles in size, for example, its length increases by 20 times 

 as much (by 100 feet) as its height (by only 5 feet). And this in- 



« Cornish, Vaughan. 1929. Waves of the sea, Encyclopedia Brittanica, 14th ed. vol. 

 23, p. 442. 



