204 HARBOUR CONSTRUCTION. 



It is, no doubfc, well to select the largest of the stone for 

 deposit on the weather slope and top of the mound; but if 

 this should involve reserving stone to any great extent, either 

 in the quarry or elsewhere, the cost of the work might be 

 much increased, inasmuch as it would necessitate the hand- 

 ling of the material twice, or oftener, and would, further, be 

 likely to cause inconvenience by congesting the quarry and 

 works. 



The quartzite stone of which the mound of Holyhead break- 

 water is composed weighs about 166 Ibs. per cubic foot, or 

 13 J cubic feet to a ton. In the mound, one ton occupies some- 

 what less than 20 cubic feet. The voids are therefore about 

 one-third of the whole mass of the mound. 



Casual reference to the shortcomings of Alderney breakwater 

 has already been made in this and in some of the preceding 

 chapters, but before proceeding further it will be well to draw 

 attention more directly to the several lessons which that great 

 work teaches. 



It may here be remarked that there is no branch of the 

 engineering profession which is so dependent upon experience 

 especially that gained by failures as the one which deals 

 with the construction of sea-works. The practical knowledge 

 which has been acquired during recent years in constructing 

 breakwaters of various types, is therefore of the greatest 

 value. 



Fortunately, those in charge of works have, as a rule, been 

 ready, not only to admit their failures, but also to give to the 

 profession the benefit of the experience gained by them. In 

 alluding to them so freely as I do, I therefore trust the reader 

 will acquit me of any wish to disparage the works of others, and 

 will recognize that my only object is to direct attention to the 

 useful lessons which they teach. 



It is not easy, in the present day, to realize the disadvantages 

 under which the engineers of fifty or sixty years ago laboured, 

 in having no works of magnitude, such as they have left us, to 

 guide them in their designs. Even now, with all our experience, 

 winter storms remind us very forcibly, almost every season, 

 that we are still far from being infallible. 



But to return to our subject. As the construction of Alderney 

 breakwater progressed, the weak points in its design became 

 manifest, and the section was altered from time to time as 



