DEPTH OF WAVE ACTION. 411 



Wave Action. 1 Waves generally are classed under two heads, i.e. 

 waves of oscillation and waves of translation, but it appears doubtful 

 whether any wave generated by the force of the wind can be 

 considered as purely oscillatory. When there is actual and con- 

 tinuous horizontal force applied, there must be actual transference of 

 water, the amount of course varying considerably with the strength 

 of the wind, and the time it has been blowing. 



That waves exert great disturbing powers at moderate depths it 

 undoubted, and that their influence is felt in deep water is tolerably 

 certain, but the nature and extent of this action in deep water is by 

 no means so well understood. 



The late Sir J. Coode stated from personal observation that the 

 strength of the Chesel Bank is disturbed by storms at a depth of 8 

 fathoms. 



Waves from 6 to 8 feet high have been seen to change their 

 colour from abrasion of the bottom after passing into water of 7 to 8 

 fathoms. 



Sir J. Douglas has stated that coarse sand has been thrown from 

 a depth of 25 fathoms on to the lantern gallery of the Bishop's Rock 

 lighthouse. He also alludes to the fact, well known amongst 

 fishermen on coasts with an Atlantic exposure, that lobster-traps 

 are occasionally filled with coarse sand in depths up to 11 fathoms. 



The late Mr. R. Stevenson held the opinion that the dispersion 

 of fishes, evinced by their disappearance from fishing-grounds in 

 stormy weather, tends to show the disturbance of the sea at a depth 

 of 30 to 40 fathoms. 



It is not unusual for vessels sunk in comparatively deep water 

 to be broken up by subsequent storms. Tor instance, a ship which 

 sunk off the Gold Stone, on the Northumberland Coast, in 11 

 fathoms, was broken up by a later gale, and part of the wreck washed 

 on shore. 



The late Sir J. Airy noted that heavy ground-swells break in a 

 depth of 100 fathoms. 



Proofs of the sea being disturbed to a considerable depth exist 

 in the fact of some large drift-stones of more than 2 tons weight, 

 having been thrown during storms from deep water on to the rock, 

 on the Scotch coast, upon which the Bell Rock lighthouse is erected. 



Formerly rubble foundations were considered safe from disturb- 

 ance at very moderate depths below low water. Now, however, it is 

 judged wiser to keep the surface of a rubble mound some 15 to 20 

 feet, and in some cases considerably more, below low-water level. 



Valuable evidence as to the degree of exposure, and the 



1 Stevenson's " Docks and Harbours; " Engineering, vol. xliv. p. 73; M.P.LC.R, 

 various. 



