380 Scientific Proceedings, Royal Dublin Society. 



Here the breakwater has assumed somewhat the character of a 

 very deeply water-logged vessel, which, however, for purposes of 

 transport, can be brought to the surface at any time. 1 Almost 

 the whole of her iron work is then open to easy inspection. As 

 shown in the figure, the water-chamber is full, and the inertia is a 

 maximum. The air-pump tube C enables the water to be expelled 

 from the hold through the valve V. This valve is commanded 

 from the upper deck, and may then be closed. The interior is 

 accessible by means of a hatch. When thus raised, the break- 

 water is easily towed from place to place. The chemical influence 

 of sea-water on iron being mainly confined to the zone between 

 wind and water, this part of the structure is of double thickness, 

 so that renewal may be effected by removing the outer plates. 

 This also secures a specially strong construction at the part most 

 tried by wave action and possibly even exposed to impact of 

 floating bodies. 



The floating part is stiffened by frequent bulk-heads. I would 

 suggest for these a spacing of 20 feet, and that similar bulk-heads 

 divide the water-chamber, spaced every 40 feet. The float is, in 

 fact, a strong box-girder, and this character, indeed, prevails 

 throughout. The amount of lateral and vertical rigidity will, of 

 course, depend upon the dimensions conferred upon the structure, 

 and the thickness of steel or iron used in its construction. I 

 assume a beam of about 36 feet on the upper deck, and a maximum 

 beam (submerged) of nearly 50 feet. The draught would, on the 

 proportions shown, then come out at about two and a-half fathoms. 

 For shallow-water coast protection this should be sufficient. In 

 this case also a length of 300 feet would be admissible : perhaps 

 rather more. 



It will be seen that the principle here involved is much the 

 same as that of the first design. The inertia is increased by a large 

 mass of water which has to rise and fall with the vessel. I have 

 made a rough computation of the masses involved. Assuming the 

 dimensions as above, and that 1-inch steel plate is used through- 

 out, and allowing twenty per cent, additional metal for girders, 

 rivets, etc., the mass of the hull, if 300 feet in length, comes out at 



1 A similar advantage can be conferred on the first design by covering in the 

 sloping edges QP by continuous plates. 



