230 HARBOUR CONSTRUCTION. 



beneath the surface by the action of the tidal current (where 

 such happened to exist), while the ties would be fitted with 

 movable collars to secure the various sections of the screen in 

 their places, and to provide against irregularities of distance. 



It was stated that the material used in the construction of 

 such screens might be varied according to circumstances, piles 

 of timber or cast iron being substituted for those of wrought iron 

 where thought to be desirable. 



The cost of a wave-screen such as this was estimated at 

 850,000 per sea mile, the value of the ironwork, delivered on 

 the works, being taken as 20 per ton. 1 



It seems evident that the stays, to be of service, would have 

 to be fixed in some more secure manner than that indicated in 

 the drawing, seeing that the rush of water towards the breast 

 of each wave, as it advanced (see Fig. 6, p. 25) would tend to 

 push the screen forward, while the impact of the waves would 

 drive it back. The stays would thus be alternately in tension 

 and compression, and the screen would be pushed backward 

 and forward by turns, in a manner that would be very trying 

 to it. 



During erection, a structure of this kind would be subject to 

 great sea risk ; and in view of the difficulties inseparable from 

 such a class of work, and the small percentage of days upon 

 which it could be carried on in exposed situations, its progress 

 would be very slow. 2 



It would thus appear that the stability of such a structure 

 would be small ; that a considerable amount of undulation would 

 be admitted into the harbour between the piles ; that its erection 

 would be slow and attended with considerable sea risk if in an 

 exposed position ; and that its cost, as estimated, would be more 

 than sufficient to construct a substantial breakwater of one of 

 the more ordinary types. Altogether, therefore, the design does 

 not seem to have very much to commend it. 



The more one considers wave-screens of the several types 

 we have been describing, whether from a theoretical or practical 

 point of view, the less desirable they seem to be ; indeed, from 

 experience in constructing iron piers and other sea- works, I feel 



1 "Report of Royal Commission on Harbours of Refuge, 1859," vol. i. p. 31. Also 

 pamphlet * On the Construction and Principle of a Wave Screen," by E. K. 

 Calver, R.N. (1858). 



2 It was proposed to screw these piles from a barge. 



