WIND CURRENTS AND WIND WAVES 149 



As already stated, these waves proceed as long gravitational waves, 

 and their velocity of progress over a uniform bottom, therefore, should 

 be equal to -x/gh. Where the depth to the bottom, /?, is variable, the 

 velocity of progress will be less than -x/gh, where h is the average depth, 

 but it has been found that the velocity of progress is smaller than should 

 be expected even if variations in depth are considered. The study of 

 the rate of propagation of these waves served, in spite of this circumstance, 

 to give an idea of the average depth of the ocean prior to the time of 

 deep-sea soundings. Thus, in 1856, Bache computed the average depth 

 of the oceans to be about 4000 m, whereas Laplace had assumed an average 

 depth of about 18,000 m. 



Destructive 'Svaves'' caused by wind and barometric pressure are of 

 an entirely different nature. In this case one has to deal not at all with 

 the effect of a wave, but, instead, with inundations that are almost 

 directly caused by the ocean waters being swept up against the coast by 

 violent storms. Abnormally high-water levels caused by strong winds 

 are frequent on many coasts, but, fortunately, the sea level rarely rises so 

 much that great damage occurs. The most destructive storm ''wave" 

 known in the history of the United States is that which practically 

 destroyed Galveston on September 8, 1900. A West Indian hurricane 

 approached the coast of the Gulf of Mexico, where, at Galveston, the 

 barometric pressure fell from 29.42 inches at noon to 28.48 at 8:30 p.m. 

 At the same time the wind velocity increased to 100 miles per hour at 

 about 6:00 p.m., when the anemometer was broken to pieces. It has 

 been estimated that the average wind velocity between 6 :00 and 8 :00 p.m. 

 must have been about 120 miles per hour. During the day the water 

 rose slowly but steadily until the wind had reached hurricane force, 

 when a much more rapid rise took place. In the evening the water level 

 was nearly 15 feet above mean high water, and large districts of the city 

 were flooded. Nearly 6000 persons were drowned, and the property 

 damage ran into tens of millions of dollars. 



The hurricane that on September 21, 1938, struck the coast of New 

 England brought an even higher water level in many localities, but did 

 not cause so much loss of life. At Buzzard's Bay the highest w^ater level 

 ranged from 12 to 15 feet above mean low water, and at Fall River it 

 was reported that ''the water came up rapidly in a great surge," rising 

 to about 18 feet above normal. Nearly 600 persons lost their lives, 

 and the property damage has been estimated at $400,000,000, but only 

 part of this damage was due to destructive waves. 



It should again be emphasized that destructive "waves" caused by 

 wind and also commonly included among "tidal waves" are not waves, 

 but represent a rise of sea level comparable to the minor changes of sea 

 level that are associated with wind and are known on all coastal areas. 



