FOR GREAT BRITAIN AND IRELAND. 107 



tions of the relations subsisting between the low-water 

 sectional areas and the amount of tidal water lying land- 

 wards of the section lines " (continues Mr. T. Stevenson), " I 

 found that in the narrow artificial channel of the River Dee, 

 Cheshire, the efficacy of a given quantity of tidal water was 

 greater than in navigations which were left more nearly in 

 a state of nature. But, as already stated, the effect of 

 excluding water being prejudicial to the lower reaches, 

 ought to lead us to other means of improvement than the 

 erection of such high bulwarks." 



"The effect of low-water training walls which do not 

 confine the strength of the current, but simply guide the 

 first of the flood and the last of the ebb-tide, are now well 

 known from their extensive employment as for example on 

 the Clyde, Ribble, Lune, Nith and other rivers." 



The proportion of backwater to the low-water sectional 

 area at the mouth or over the bar of any navigation will 

 depend on the nature of the bottom, the force and direc- 

 tion of the waves, and the direction in which the stream 

 enters the sea. The proportion for four navigations is 

 as under : 



RATIOS OF LOW-WATER SECTIONAL AREAS OF RIVERS AT THEIR 

 JUNCTION WITH THE SEA, AND THE TIDAL CAPACITY OF 

 THEIR ESTUARIES, AND DISCHARGE OF FRESH WATER. 



Boyne . . . 29*7 sq. yds. to 1,000,000 cub. yds. of backwater. 



Tay . ... 27-4 



Mersey ... 22-5 



Montrose . . 28*0 



ARTIFICIAL SCOURING. 



On this subject the remarks of Mr. Stevenson are very 

 important. 



" The preservation of the depth of harbours at a lower 



