METHODS OF CONSTRUCTING BREAKWATERS. 245 



of cost will, however, always be an important factor in deciding 

 all such matters, and this will be affected by the quantity of 

 work to be done per unit of length, seeing that a small mound 

 might require almost as costly a staging as a large one. 



During the construction of the Portland breakwater, the 

 work of depositing rubble from the staging was, on an average, 

 not interrupted, on account of weather, more than from two to 

 three days in a year. 



In the preparation of foundations, and the setting of blocks, 

 especially under water, staging offers many facilities. 



Work in its various stages may be carried on simultaneously 

 over a considerable length, several machines being employed as 

 against one in over-end work. The overhead plant requisite for 

 dealing with heavy loads may, moreover, be of a comparatively 

 inexpensive character. Temporary platforms for the use of 

 divers, or for other purposes, may be suspended from the stage 

 in almost any position, and the lines and levels of the work may 

 be given and checked with the greatest ease. 



Staging is, however, for the most part costly. That used in 

 constructing the Portland breakwater cost about Sd. per ton of 

 rubble deposited. At Holyhead it cost about 20 per lineal 

 foot of breakwater, or 5cZ. per ton of stone deposited. Its use, 

 moreover, almost always involves considerable sea risk, and if 

 the work should be long in hand, the teredo and other sea-pests 

 will have to be reckoned with. The advantages of staging, are, 

 however, so great, that they will, in many instances, be found to 

 outweigh its shortcomings. 



Barges. When hopper-barges are employed for depositing 

 rubble in the sea, shoots, down which the stone is tipped from 

 suitably constructed waggons, are usually fixed at the loading- 

 berths. When, however, the stone is of large size, it is necessary 

 to employ cranes for loading it, in order to prevent injury to the 

 barges by the falling of such heavy weights. In such cases it 

 is convenient to have the bodies of the waggons so arranged that 

 they may be lifted off their under-frames. They thus form 

 large skips, which, together with their contents, may be lowered 

 into the barges, the stone being discharged by means of a 

 tripping-chain. In this way the slinging of stones by means 

 of chains is avoided, and much time is saved. The size of 

 barges commonly used for this work varies from about 60 to 

 120 tons. 



