260 HARBOUR CONSTRUCTION. 



concrete is subjected by the moving rails and the loose canvas 

 is pitiable to see. In such circumstances I have sometimes 

 wished that a wave would clear all the rails and canvas away 

 and leave the concrete to take care of itself. 



On one occasion I covered the canvas with boiler-plates 

 f inch thick, feeling confident that they would not be displaced 

 as they possessed considerable weight, and there appeared to be 

 nothing for a wave to take hold of. On the following morning, 

 however a moderate sea having got up during the night I 

 found that they had not only been displaced, but that some of 

 them had been carried away altogether. 



At low water there is frequently a considerable interval of 

 " slack," during which the water-level remains almost stationary. 

 It follows, therefore, that newly mixed concrete put in at or near 

 that level will be subjected to an undue amount of fretting by 

 the waves. The same might happen at or near high water, but 

 work put in at that level can generally be allowed more time 

 to harden, before the water reaches it, than it is possible to give 

 to work near the low-water level. At intermediate levels, 

 excepting in places where the tidal range is very small, the 

 water, after reaching the work, soon covers it, and the fretting 

 action of the waves is thus quickly reduced. Even a single foot 

 of water over new work seems to have a magical effect in check- 

 ing the chafing action of small waves. 



Notwithstanding the foregoing facts, it is not uncommon, 

 in designs for sea-works, to find mass-work stopped within a 

 few inches of low water of spring tides, for the bedding of what 

 is called the "low- water course." Experience, however, soon 

 teaches those who have the practical carrying out of such works 

 that it is well to have as little to finish off at that level as 

 possible. 



Excepting at spring tides, work so situated cannot be pro- 

 ceeded with at all, and should it happen that rough weather 

 coincides with these tides, the work must wait until a more 

 favourable opportunity presents itself possibly a fortnight 

 hence, but frequently very much longer. 



The time occupied in the execution of work, and also its cost, 

 are often much increased from this cause. 



In bringing up work from below low water, it is best, for 

 the reasons already stated, either to finish off the mass-work 

 and form a level bed several feet below the low-water level 



