60 HARBOUR CONSTRUCTION. 



necessitates extra good work, and the adoption of precau- 

 tionary measures to enable it, while " green," to withstand the 

 wash of the sea. 



On the other hand, work within the tidal range may be 

 executed without the aid of divers ; so that, in some cases, the 

 facilities which range of tide affords for executing work below 

 the mean sea-level if that datum be assumed instead of low 

 water may, in a measure, compensate for the extra cost which 

 it involves above that level. 



Notwithstanding what has been stated in reference to the 

 prejudicial effect of tidal range under certain conditions, particu- 

 larly in regard to the construction of breakwaters, it is scarcely 

 necessary to say that the importance of tides is such that they 

 constitute the power upon which, when properly directed, 

 depends the very existence of the great majority of navigable 

 river-channels. 



Numerous creeks and small bays are also, by tidal range, 

 rendered available as harbours for the smaller class of shipping, 

 more particularly for that class which is engaged in the fishings. 

 Sometimes such bays or creeks are naturally protected by 

 outlying rocks, and in other cases shelter may be afforded, at a 

 moderate cost, by taking advantage of natural facilities. 



Fishing-boats and other small vessels enter these harbours at 

 high water, or whenever there is sufficient water to admit of 

 their doing so ; and as the tide recedes they settle down upon, 

 let us hope, an even, soft bottom, and lie secure, unless, as some- 

 times happens, they are ruthlessly disturbed by the "run" 

 caused by storm- waves entering the harbour. 



The great drawback to tidal harbours is that they are not 

 available for purposes of refuge at all times of the tide a fact 

 which often entails very serious loss of life and property. 



Judging, however, from their number, and from the additional 

 harbours of this class which have been constructed of late years, 

 and are still being constructed, it is evident that the shelter and 

 accommodation which they afford, imperfect as it may be, is 

 appreciated. 



Many docks, including graving docks, are entirely dependent 

 upon tidal range for their working, their sills being often laid at 

 or above the level of low water ; and but for tidal range, those' 

 useful structures called "gridirons," whereon vessels are so often 

 conveniently cleaned and repaired, would have no existence. 



