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XXXIY. 



ON FLOATING BEEAKWATERS. 



By J. JOLY, Sc.D., F.R.S. ; 



Professor of Geology and Mineralogy in the University of Dublin ;. 



Hon. Sec, E.D.S. 



(Plates XXXV. and XXXVI.) 



[Read, May 16 ; Received for Publication, May 19 ; Published, July 5, 1905.] 



The subject of floating breakwaters has engaged the attention 

 of engineers from time to time, and references to experiments 

 and suggestions (mainly the latter) will be found in treatises on 

 harbour engineering, in the Report of the Commissioners of 

 Harbours of Refuge, which dates back to the middle of the last 

 century, as well as in the Transactions of the Institution of Civil 

 Engineers. 



In none of these sources of information have I been able to find 

 any reference to the principle involved in the suggestion described 

 in the following note. It formed the subject of an informal 

 communication to a Trinity College Society some years ago. 

 Recent references in the daily papers to the proposed improve- 

 ments of harbours on the east coast of Ireland, induce me to publish 

 it. The conditions, both in the case of Arklow and Wicklow, seem 

 precisely those which would justify its use. 



It is evident that if a floating body is to check the transmission 

 of undulating motion in water, its own oscillations in the water 

 must be damped to such a degree that only vibrations, as a whole, 

 of long period are open to it. Thus a very large ship will give 

 rise to still water under its lee, provided the sea is not so great as 

 to carry the vessel up and down upon its undulations. 



Let us now imagine a vessel riding in a sea sufficiently big to 

 raise her upon each wave. Such a sea is not materially obstructed 

 by its passage across the vessel (which we will suppose broadside 

 on to the waves), although broken water will not exist under her 

 lee. Let us now assume a horizontal platform to be attached 

 beneath the vessel sufficiently far down to be in still water. 

 If the platform and its attachments are strong enough to 



