66 SEAS AND SWELLS 



trochoid profile characteristic of the so-called free wave'" of theory. 



Meantime, the individual wave crests, that are seldom more than a 

 few times as broad transversely as the wave is long during windy 

 weather, tend to expand farther and farther sidewise, while the nar- 

 rowest of them seem also to be obliterated in some way, until finally 

 a crest that was only 500 feet or so wide, while the gale was still blow- 

 ing, may expand to a breadth of 1,500 to 2,000 feet or even more. We 

 have ourselves observed swells that were well over one-half mile wide 

 just before they broke upon the shore. But the variations in the 

 lengths of successive members of a given train of swells persists as 

 the swell proceeds. Among 139 nearly consecutive breakers, for ex- 

 ample, the periods of which were timed on the south coast of England 

 after a heavy gale, the shortest, with a period of 10 seconds, was only 

 0.385 as long as the longest, with a period of 26 seconds (Cornish, 

 1910, p. 89). 



Theories have been developed according to which the lengths of 

 waves from crest to crest, and hence their velocities and also their 

 periods, should increase after a sea has been transformed into a swell, 

 contradictory though this might seem at first sight, when we remember 

 that it is from the wind that the waves have derived their whole 

 energy in the first place (Sverdrup, Johnson, and Fleming, 1942, 

 p. 534) . This is supported by the fact that swells of very long periods 

 reach the Calif ornian coast (p. 36) and also the Moroccan coast (p. 69) , 

 more frequently than would be expected from the frequency of storms 

 in the North Pacific and the northwest Atlantic, severe enough to pro- 

 duce such long w r aves. And while it has been questioned whether the 

 periods recorded for any particular series of breakers have been 

 longer in any known case than might possibly have accorded with 

 the maximum strength of the fiercest squalls during the gales that had 

 set them running, 23 we believe the view is correct, that swells do gain 

 in length, and in velocity and probably period as well, as they advance 

 farther and farther from the regions of their birth. 



The alteration of seas into swells is a progressive event; hence, it is 

 never possible to pick a precise moment prior to which the waves are 

 of the former character, and subsequent to which they are purely of 



22 A "free wave" may be defined as one that is set in motion by a sudden impulse acting 

 onee and for all and that owes its continued existence solely to the force of gravity. 



23 The offshore velocity (67.5 knots, or 78.5 statute miles per hour) deducible from the 

 longest periods yet recorded for any group of breakers on any coast (average, 22.5 seconds 

 for 12 successive breakers on the south coast of England) was about 11 statute miles per 

 hour lower than the probable maximum velocity, in gusts, of the wind during the particular 

 Atlantic gale that Keneiated them. Wind velocities, in gusts, of 80 to 90 statute miles per 

 hour were, in fact, recorded (by anemometer) during the preceding month over southern 

 England, during several trusts, with a maximum of 101 miles per hour, briefly. (Cornish, 

 1910, pp. 118-120. and 1934, pp. 11-14.) But it is a question whether squalls as violent 

 as these last Ion;; enough, or extend over areas large enough, for the generation of waves 

 as long as those of the group of swells in question. This matter of squalls is also discussed 

 on pages 20 and 26. 



