Principles of training Rivers through Tidal Estuaries. 511 



result from the concentration of the tidal rise by the sudden contrac- 

 tion of the estuary above Quillebeuf. The period given to each tide 

 in working was about 25 seconds, which appeared fairly to reproduce 

 the conditions of the estuary.* After the model had been worked for 

 a little time, the channels near Quillebeuf assumed lines resembling 

 those which previously existed ; and a small channel appeared on the 

 northern shore, by Harfleur and Hoc Point, which is clearly denned 

 in the chart of 1834. The main channel also shifted about in the 

 estuary, and tended to break up into two or three shallow channels 

 near the meridian of Berville, where the influences of the flood and 

 ebb tides were nearly balanced. The model, accordingly, fairly repro- 

 duced the conditions of the actual estuary previous to the commence- 

 of the training walls; though the channel in the estuary did not 

 attain the depth, as represented by the proportionately large vertical 

 scale, which the old channels possessed, owing, doubtless, to the com- 

 paratively small scouring influence which the minute currents in the 

 model possess. The sand, in fact, cannot be reduced to a fineness 

 corresponding to the scale of the model, whilst the friction on the 

 bed is not diminished equivalently to the reduction in volume of the 

 current. Silver sand had been used on account of its being readily 

 obtained, its purity, and absence of cohesion, as it was hoped that the 

 water, by percolating freely through it, would more readily shift it. 

 A film, however, seemed by degrees to form over its surface, reducing 

 considerably its mobility ; and as the action of the water on it con- 

 sisted merely in rolling the particles along the bottom, this sand did 

 not prove satisfactory for producing the requisite changes when the 

 training walls were inserted in the model. It became, therefore, 

 essential to search for a substance which the water could to some 

 extent carry in suspension for a short period. 



Trial of Various Substances for Forming the Bed of the Estuary. 

 Some substance was required, not necessarily sand, insoluble in water, 

 easily scoured, and therefore not pasty or sticky, and sufficiently fine 

 or light to be carried in suspension to some extent by the currents in 

 the model, and not merely rolled along the bottom like the silver sand. 

 A variety of substances of low specific gravity, and in powdered form, 

 were accordingly tried in succession during the first half of 1887. 

 Pumice in powder proved too sticky ; and flower of sulphur was too 

 greasy to be easily immersed in water. Pounded coke was too dirty 

 to be suitable, and particles of it floated. Yiolet-powder became too 

 pasty in water ; and fuller's earth and lupin seed exhibited similar 

 defects. The grains of coffee grounds were too large in water, and 

 moved up and down in the currents too readily ; whilst fine sawdust 



* According to the formula in the paper by Professor O. Eeynolds, on his Mersey 

 model, read at the Frankfort Congress in August, 1888, the tidal period would be 

 nearly 23 seconds. 



