520 Mr. L. F. Vernon-Harcourt. 



channel had a good width at low water throughout, in spite of the 

 distance apart of the training walls in the model, the whole channel- 

 being below low- water level, except near the southern wall between 

 Berville and Havre, and against the northern wall nearly opposite Hoc 

 Point, where banks emerged slightly above low water. The channel, 

 moreover, was distinctly, though slowly, improving with the con- 

 tinuance of the working, and the banks diminishing. There was also 

 a fair depth in the channel, the shallowest place being opposite 

 Berville, whilst a deep place was formed just above, near the southern 

 wall between La Roque and Berville. The depth in all the outlet 

 channels was well maintained ; and though deposit naturally took 

 place behind the northern training wall, no accretion was visible 

 along the foreshores outside. 



Considerations affecting Experimental Training Works. 



The value of experiments resembling those just described depends 

 entirely upon the extent to which they may be regarded as producing 

 effects approximately corresponding, on a small scale, to those which 

 training works on similar lines, if carried out in an estuary, would 

 actually produce. If the effects of any training works could be 

 foreshadowed by experiments in a model, the value of such experi- 

 ments, in guiding engineers towards the selection of the most suitable 

 design, could not be overestimated. 



Some of the influences at work in an estuary cannot possibly be 

 reproduced in a model such as winds and waves. Winds coming 

 from different quarters are variable in their effects ; but the direction 

 of the prevailing wind indicates the line in which the action of the 

 wind has most influence, which may be exerted in reinforcing the 

 flood or ebb currents, and may aid or retard accretion by blowing the 

 silt-bearing stream more into or out of the estuary. Waves are the 

 main agents in the erosion of cliffs along open sea-coasts, and in 

 stirring up sand in shallow places ; and the material thus put in sus- 

 pension may be transported by tidal currents, aided by wind, into an 

 estuary, and be deposited under favourable conditions. These circum- 

 stances affect the rate of accretion, which cannot be investigated 

 experimentally, as it is impossible to reproduce in a model the propor- 

 tion of silt in suspension, which, moreover, varies in any estuary with 

 the state of the weather and tide, and the volume of fresh water 

 discharged. Inside an estuary, also, waves in storms may erode the 

 shores at high tide, and modify the low- water channels; but the first 

 effect is very gradual, and the second is intermittent only occasionally 

 occurring. 



The main forces acting in any tidal estuary are the tidal ebb and 

 flow and the fresh- water discharge, which are constantly at work ; and 

 they regulate the size of the channels in an estuary, and for the most 



