FOR GREA T BRITAIN AND IRELAND. 105 



foundation of straight embankments diverging slightly 

 towards the ocean. 



" The low-water sectional areas of many estuaries and of 

 creeks increase directly as the quantities of tidal water which 

 lie landwards of each section-line? 



The great desideratum required, before any correct theory 

 can be formed upon this subject, is the means of discovering 

 the scouring power of a given quantity of water. The daily 

 varying amount of tidal water which is propelled into an 

 estuary from the ocean, the ever-changing amount of the 

 land waters, which is sometimes extremely small and at 

 others enormously increased in volume, and the extreme 

 diversity of materials forming the bed which, at different 

 parts of the bed of the same river, consist of gravel, sand, 

 or clay, exhibiting endless variety in the sizes and coherence 

 of the particles present an almost hopeless perplexity for 

 the mind to grapple with. To such causes we may 

 attribute the conflicting opinions which are daily hazarded, 

 and no one of which admits of being proved or disproved. 

 From a comparison made several years ago of different low- 

 water sections of the estuaries of various rivers, and also of 

 bays and creeks in a state of nature, " I found," says Mr. 

 Stevenson, "that the low-water sectional areas increased 

 directly as tJie quantities of tide-water tJtat lay landwards of 

 each section-line'' " By thus comparing the sectional area at 

 any point with the area at a point farther down the estuary, 

 we free the question of the difficulty of dealing with the 

 unknown action of the freshes which is nearly the same at 

 both places, as well as of the ever- vary ing amount of the 

 tides, which is also very nearly equal at both." 



"The difference of area will probably in all ordinary 

 channels depend on the quantity of tidal water contained 

 between the two places where the sections are taken. We 



