Joly — On Floating Breakwaters. 379 



constitute, along with the ship, a rigid system, and if the platform 

 is of sufficient area, the upward and downward motion of the 

 vessel will be completely checked. She will no longer allow the 

 wave-motion to be so freely transmitted. A wave rising against 

 the side of the vessel, in order to lift the vessel has to set in motion 

 the mass of water surrounding the platform. The vessel, therefore, 

 instead of facilitating the transmission of the wave-motion by 

 rising with the wave, obstructs the passage of the latter to the full 

 extent of her draught. The wave will accordingly be scattered 

 and reflected in a greater degree, its energy being expended partly 

 in turbulent motion, partly in vibrations communicated to the 

 vessel, and in part sent back in the reflected wave. 



Such a breakwater is a floating body towards the tidal rise and 

 fall of water-level, but not to the more rapid rise and fall of waves. 

 In a sense, therefore, it is as fixed in position as a stone breakwater 

 rising from the bottom. There is no doubt it can be made to be 

 so by conferring sufficient dimensions upon it. These dimensions 

 need not be extravagant where the conditions are not such as to 

 require protection from deep-water waves. The principle is 

 embodied in its most generalized form in Plate XXXV., show- 

 ing in cross-section a cylindrical float supporting a submerged 

 platform PP at a depth of about four fathoms. The float 

 and platform are braced together by vertical webs at frequent 

 intervals. A submerged web, transverse to these, further connects 

 the float and platform, and stops the transmission of undulatory 

 motion between float and platform. Assuming a diameter of 

 30 feet for the float, and taking into account the lateral horizontal 

 rigidity conferred upon it by its attachment to the platform (which 

 is about 60 feet wide), a length of ten diameters would probably 

 be admissible. Such a breakwater, moored in some six fathoms of 

 water off a bight or small indentation of a coast, would give shelter 

 within, allow entrance at either extremity, in no way obstruct 

 tidal currents or tidal scour, and, lastly, be removable at pleasure if 

 circumstances so warranted. Obviously, a construction involving 

 less or more draught might be conferred upon it according to 

 requirements. 



A modification of this breakwater, presenting different features 

 of construction, and doubtless having advantageous qualities, is 

 shown in Plate XXXVI. 



