148 



KNOWLEDGE. 



[JtJLY, 1901. 



in the southern part of the China seas, dm-ing a strong 

 N.E. gale of several days' duration, were 21 feet in 



1 



t=< 



height. This was off Cape Varella. with a length run 

 of about 800 miles. Probably the scale of the cyclonic 



disturbances is larger in the gi-eat oceanic areas where 

 meteorological conditions are uniform over large areas. 

 This would give a gi-eater real length of fetch to wind of 

 a given direction and a longer period of operation. The 

 waves of violent storms in the oceans mav be reckoned 

 to be one-half as high again as those of the seas. I take 

 provisionally 20 feet average and 30 feet occasional 

 single waves for the seas, and 30 feet average with 45 

 feet occasional single waves for violent storms in the 

 oceans, values which in high southern latitudes are pro- 

 bably somewhat exceeded. This difference I ascribe 

 partly to the large scale of the weather in the oceans, 

 with increased length of fetch and increased duration of 

 wind. 



Let us consider separately the contribtiting cause, 

 greater length of run of the waves. It is the shortness 

 of the principal waves first formed bv wind which limits 

 their height. The increase in length of the dominant 

 form is a gradual process involving the destructive inter- 

 ference of the shorter waves, and the longer waves must 

 attain a greater amplitude than the shorter before they 

 become conspicuous in the presence of the latter, as I 

 have already explained. Not only does it take a con- 

 siderable time and length of fetch to make the longer 

 waves the dominant form in a gale, but since they travel 

 more swiftly than the short ones they sooner reach the 

 lee shore. Thus in enclosed seas they soon reach the 

 margin and are put out of action. 



In lakes very long waves, such as move with nearly 

 the velocity of the wind in a strong gale, would be too 

 flat to be visible, even after the wind has ceased, except 

 )jerhaps under a very low angle of illumination, and this 

 flatness would prevent their breaking on the shore, 

 though the}' might perhaps produce a welling up and 

 down in a period of abotit 15-18 seconds for the complet« 

 double oscillation. 



On the other hand, longer waves preserve their form 

 for a much greater time than short ones, as they travel 

 on after the cessation of wind, the rate of flattening 

 being theoreticalh^ inversely proportional to the square 

 of the wav3 length. Thus in the oceans, when a storm 

 has occuiTed far from a lee shore, the shorter waves die 

 out by subsidence before the longer waves run out, and 

 the sea between the place where the waves were raised 

 and the lee shore is left heaving for days with a long 

 swell. This effect is best seen where the ocean encircles 

 the globe in high southern latitudes. Here the storms 

 come from the westward, the strongest winds are 

 westerly, there is always a swell running from the west- 

 ward, and there is no eastern shore, the length of iTin 

 of the waves being practically infinite on the re-entrant 

 surface of the circumpolar water. As each westerly gale 

 rises it blows over the ridges of the heavy swell already 

 iiinning, pressing strongly on the subsiding back, and 

 opposing less the rising front, increasing thus the height 

 of each billow. Thus in the swelling ocean the storm 

 waves of 400 to 600 feet do not have to wait so long 

 before attaining their maximum steepness and becoming 

 the dominant fomi as if they had to be slowly developed 

 by the tedious process of destructive evolution from the 

 short waves first formed when smooth water is ruffled 

 by the wind. 



Marked as is the difference between the storm waves 

 of the Mediterranean, or other seas, and those of the 

 oceans, it is the swell which comes in upon the shore 

 facing the open ocean which we instinctively recognise 

 as the true index of a vaster expanse of water. These 

 swells, as I have pointed out, must have, in deep water, a 

 speed but little inferior to that of the wind during galos. 



