16 WAVE DIMENSIONS 



The last of these three problems has received the most attention, 

 no doubt because exceptional and spectacular phenomena are naturally 

 the most impressive, especially if they involve imminent danger to 

 human life and property, as really large storm waves do. But the 

 heights of the common run of waves is of equal or greater importance 

 from the practical standpoint, because the seaman has to do with 

 these every day that he is at sea, but may never, in a lifetime, encounter 

 waves of the great heights that are sometimes reliably reported, even 

 if his voyages regularly cross and recross the stormier parts of the 

 ocean in stormy seasons. The relationship that the heights of waves 

 bear to the wind is also of concern, not only from the theoretical stand- 

 point, but very directly from the practical, as indicating the general 

 dimensions of the waves that are to be expected in different parts of 

 the ocean and at different seasons, according to the prevailing condi- 

 tions of the weather. And discussion of this phase of the problem 

 is the logical introduction to any account of the heights of the waves 

 that ships do actually meet. 



The heights of waves are determined by the strength of the wind, 

 combined with the length of time during which a wind of any given 

 force may have been acting on them. It is a matter of common 

 knowledge that high winds do not generate high waves instantaneously, 

 but require a considerable period to do so. Since the waves are con- 

 stantly advancing, meanwhile, the time during which the wind may 

 have been acting upon them is proportional to the distance that they 

 have run, or to the "fetch" as this is termed. And it is for this reason 

 that the sea is always smooth under the windward shore, no matter how 

 strong the wind nor how long it may have been blowing, with the 

 waves increasing in height out from the land. In other words, large 

 waves can develop only in comparatively broad bodies of water. The 

 rate at which waves gain in height, under winds of different strengths, 

 has been much discussed and the values given in table 3 are taken from 

 one of the most authoritative attempts that has been made to discover 

 empirically the maximum heights of the waves that winds of different 

 strengths ordinarily produce (given sufficient time and fetch). 



The growth of waves in relation to the wind has also been subjected 

 to theoretical analyses. These yield the most trustworthy information 

 now available as to the rates at which waves grow in height under 

 winds of different strengths, for the fact that waves are constantly ad- 

 vancing has so far prevented any one from bringing the growth of a 

 wave under close observation. 



The theoretical relationship between height of wave, strength of 

 wind, and duration and fetch of the latter is summarized in tables 

 4 and 5. 



