DEPTH OF MOVING WAVE. 65 



or about ISJ miles per hour ; a wave of 674 feet, moving over the 

 surface of a sea 1,610 fathoms deep, travels more than 69 feet per 

 second, or nearly 50 miles per hour ; this last figure may be con- 

 sidered as an average speed for storm-waves in great seas. Since, 

 therefore, we can by calculation infer the velocity of waves from 

 their width and the known depth of the ocean-bed, it is easy to 

 determine by an inverse operation what is the depth of the ocean 

 itself, provided that we know the rate of motion of the waves. It 

 is by this method that the mean depths of the South Atlantic and 

 of the Pacific Ocean between Japan and California have been cal- 

 culated.* 



Natural philosophers have frequently discussed, the question of the 

 movement of the waves in a vertical direction. To what depth in 

 the abysses of the sea does the action of the superficial wave penetrate, 

 and at how many fathoms can it disturb the sand and debris at the 

 bottom ? It was formerly admitted, as a well- ascertained fact, but 

 without proof, that the agitation of the sea ceases to be felt at 4 to 

 6 fathoms below the surface. Direct observations made by seamen 

 in many latitudes have shown that this opinion is erroneous. Sailors 

 have frequently seen the waves break at 10, 16, and even 27 fathoms 

 of depth over hidden rocks, which proves that the rocks were ob- 

 stacles which abruptly barred the advance of the lowest part of 

 the wave. Still more frequently, during violent tempests, the water 

 has been seen charged with clay and mud, which had been raised 

 from the bottom at 50, 80, and even 100 fathoms below the surface. f 

 The direct experiments of Weber on the movements of waves have 

 likewise proved that each wave extends its influence in a vertical 

 direction to 350 times its height. Thus every wave of about a foot 

 in height makes itself felt on the bed of the North Sea at a depth of 

 50 fathoms ; whilst every oceanic billow of 33 feet is felt at about If 

 miles. It is true that at these enormous depths the action of the 

 wave is, so to speak, imaginary, for below the surface it decreases in 

 geometrical proportion ; but at about 25 to 50 fathoms only, the sub- 

 marine waves have still great force, and we can easily understand 

 that when thousands of them are abruptly arrested by submarine 

 rocks, and on the rapid slopes of sand-banks, violent eddies must be 

 produced which afterwards returning to the surface of the water 

 cause " heavy swells." From these causes arise those turbulent seas, 

 which ships encounter at times in calm weather, especially in the 



* See above, p. 17. 

 t Cialdi, Sul moto ondoso del mare, p. 174. 



