WAVES 215 



We have seen that the higher the wave with a given length, the 

 sharper the crest, or, what amounts to the same thing, the increase 

 of length without or with but moderate increase of height flattens 

 the crest, and this flattening lessens the destructiveness of the 

 waves in the open sea. In sharp-crested waves the crest water 

 "tends to roll forward faster than the front is built up, and this 

 tendency is increased by the forward brushing of the wind. Sharp 

 waves of moderate height break in 'white caps' ; great seas gain 

 curling or combing crests, which capsize small boats and break 

 with dangerous force on the decks of large vessels." (Davis- 

 17-124.) 



The Swell or Ground Szvell. "Waves spread rapidly from the 

 gales in which they are formed. As they advance they decrease 

 in height, but retain length, velocity and period unchanged. Their 

 long, flat undulation is called 'swell.' [German, Diinung; French, 

 houle.] It may swing for thousands of miles across the ocean, 

 fading as it goes. The glassy water of calm weather in the equa- 

 torial 'doldrums' is always slowly heaving and sinking with passing 

 swells." (Davis-iy: 124-j.) The swell or ground swell of great 

 hurricanes may break with destructive force on a coast a thousand 

 miles or more distant. Davis states that "landing in the harbor 

 of St. Helena is sometimes impossible on account of surf from 

 swell that originates in winter gales in the North Atlantic, thou- 

 sands of miles away." (1^6.) 



Swells are much longer than true waves. The longest were 

 measured by the French Admiral Mottez in the Atlantic, just north 

 of the equator, and had a length of 824 meters (2,703.5 feet), a 

 period of 23 seconds, or a velocity of 35.8 m. (117.5 feet per sec- 

 ond). 



On the coast of Ascension Island. Buchanan (6:2^4) observed 

 in March, 1886, a rhythmic breaking of the swell at intervals of 

 16 seconds, this being the period (t) of the swell wave. These 

 great "rollers" represented waves originating south of Newfound- 

 land, and their length after traveling this great distance is deter- 

 mined from their period to be 400 meters. At Bournemouth, on 

 the south coast of England, Dr. Cornish (10) found the mean 

 period of continuous series of 139 breakers to be 19.35 seconds, cor- 

 responding to a wave length of 585 meters. Breakers with periods 

 of 15 to 17 seconds are common at certain seasons of the year on 

 the Channel and the Atlantic coast of England and Ireland, where 

 they are known as "Death Waves" and regarded as indications of 

 coming storms. The origin of these waves is in the Gulf Stream 



