WA VES 29 



east coast of England, wave action was felt (as indicated by 

 discoloration of the water) to a depth of 12 fathoms. 



Mr. Murray mentions that shingle and chalk ballast which 

 would have a high specific gravity thrown overboard from 

 ships off Sunderland, at a distance of from 7 to 10 miles from 

 shore, into water of at least 10 fathoms in depth, were brought 

 ashore in large quantities during violent storms, and cast upon 

 the beach by wave action. 1 



It is known that waves frequently break along the edge of 

 the Bank of Newfoundland, in consequence of the depth of water 

 there abruptly decreasing to about 80 fathoms. These waves, 

 rolling in from the Atlantic, probably have a length of at 

 least 1000 feet. 



It will be evident, from what has been stated, that storm- 

 waves, when traversing such comparatively shallow water as it 

 is possible to construct sea-works in, must all, in a greater or 

 less degree, be waves of translation ; and, further, that the 

 higher and longer waves are, the more dangerous they become, 

 not only by reason of their increased volume, but because their 

 translatory nature is proportionately more developed, and the 

 forward motion of the water being that which chiefly affects 

 break waters increased . 



Small waves of oscillation may be seen dancing harmlessly 

 up and down against the vertical wall of a breakwater or other 

 upright face, whereas larger waves in the same position, having, 

 by reason of their feeling the bottom, become waves of trans- 

 lation, strike the face with great violence, the measure of which 

 is indicated by the volumes of water which are thrown up. 



Seeing that the character of waves is thus changed for the 

 worse as they proceed over a shoaling bottom, it follows that 

 the long foreshores formed by the deposit of rubble mounds, as 

 in the case of the Holyhead, Portland, Plymouth, and other 

 breakwaters of similar design, cause this transformation to take 

 place. In the two first-named breakwaters (Plymouth break- 

 water has no superstructure), the rubble mounds are for the 

 most part carried up to or above the level of high water of 

 spring tides. The waves therefore break upon the mound, and 

 are to a great extent spent, so that they do not, in their solid 

 form, reach the superstructure. 



The height to which the rubble mound is carried in a 

 1 Min. Proc. Inst. C.E., vol. xix. p. 670. 



