174 HARBOUR CONSTRUCTION. 



A similar wave of depression is propagated downwards, 

 which tends to scour out a hollow along the base of the 

 work. 



Waves which are eminently translatory throw their full 

 weight against such surfaces, and expend their energy by raising 

 columns of broken water to a great height. 



In Chapter II., I endeavoured to show that in water which is 

 not very deep in proportion to the length of waves by which 

 I mean any depth in which breakwaters have been, or are likely 

 to be constructed the higher and longer the waves are, the 

 more is the element of translation developed in them. 



It therefore follows that all great storm-waves, traversing 

 water of such depth as to admit of breakwaters being constructed 

 in it, are decidedly translatory ; and it is thus evident that a 

 vertical-faced work cannot be regarded as a charmer of such 

 waves, however well it may reflect those of smaller dimensions. 

 Hence we find vertical sea-cliffs being assailed with terrific 

 violence, and scarcely a winter passes without our hearing of 

 injury having been done to piers of the vertical type on different 

 parts of the coast. 



A vertical face, therefore, fails to uphold all the good qualities 

 which are so often claimed for it, just at the time when a display 

 of them would be most acceptable. 



About a century ago Captain Cook reported 



"At eight o'clock saw an island of ice to the westward of us, 

 being then in latitude of 50 40' south, and longitude 2 0' east of 

 the Capo of Good Hope. ... I judged it to be about 50 feet high and 

 half a mile in circuit. It was flat at the top, and its sides rose in a 

 perpendicular direction, against which the sea broke exceedingly nigh. . . . 

 In the evening we had three islands of ice in sight, all of them large ; 

 especially one, which was larger than any we have seen. The side 

 opposed to us seemed to be a mile in extent; if so, it could not 

 have been .less than three in circuit. It could not be less than 100 

 feet high, yet such was the impetuous force and height of the waves 

 which were broken against it, by meeting with such a sudden resistance, 

 that they rose considerably higher." ] 



1 " Voyage round the World." Quoted by Wright in " The Ice Age in North 

 America," pp. 103, 104. 



