766 



HISTORY OP THE SEA. 



is not rapid j but slow. The astronomer Aireysays that every 

 wave 100 feet wide, traversing a sea 164 fathoms in average 

 depth, has a velocity of nearly 2,100 feet a second, or about 

 fifteen and one-half miles an hour ; a wave 674 feet, moving 

 over a sea 1,640 fathoms deep, travels more than 69 feet a 

 second, or nearly fifty miles an hour, and this last calculation 

 may be taken as the average speed of storm waves in great 

 seas. As, therefore, we can calculate the velocity of waves 

 from their width and the known depth of the sea, we can cal- 

 culate the depth of the sea from the known size and velocity 

 of the waves. By this method the depth of the Pacific be- 

 tween Japan and California has been calculated from the size 

 and speed of an earthquake wave, which was set in motion by 

 an eruption in Japan. The accuracy of the calculation was 

 afterward established by actual soundings. 



It was formerly supposed that the disturbance of the waves 

 did not penetrate the depth of the water, below four or six 

 fathoms, bat this has been found, on further observation, erro- 

 neous. Sand and mud have been brought up from a depth of 

 a hundred fathoms below the surface, and experiments have 

 shown that waves have a vertical influence 350 times their 

 height. Thus a wave a foot high influences the bottom at a 

 depth of 50 fathoms, and a billow of the ocean 33 feet high is 

 felt below at a distance of 1 f miles. At these great depths the 

 action of the wave is perhaps imaginary, but to this reason we 

 can ascribe the heavy swells which are often so dangerous. A 

 hidden rock, far below the surface, arrests some moving wave 

 and causes an eddy, which, rising to the surface, produces the 

 " ground swells" which suddenly rise in the neighborhood of 

 submarine banks and endanger ships. This cause also explains 

 the tide races, which, coming from the depths of the ocean, 

 advance suddenly upon the beaches, destroying all that opposes 

 them. It is this cause which makes the position of light- 

 houses upon certain reefs so dangerous. The Bell Rock house, 



