732 



classifications, from the length of the course, or 

 the breadth of the mouth of rivers, are extremely 

 arbitrary. The rivers of the British islands are 

 terminated by gulfs or lakes of fresh water, in 

 which the tides cause swellings and periodical 

 oscillations ; and remind us sufficiently, that we 

 must not judge of the importance of an hydraulic 

 system * merely from the extent of the mouths 

 of rivers. Every idea of relative magnitude fails 

 in precision, if we cannot compare the volume of 

 the waters, ascertained by the measurement of 

 the velocity, and the area of the transverse sec- 

 tions-}-. It is to be regretted, that measure- 

 ments of this kind require facilities, which soli- 

 tary travellers can scarcely procure; that, for 

 instance, of sounding the whole bed of a river, 

 and of sounding it at different times of the year 

 Rivers of great apparent breadth having basins 

 of little depth, and traversed by several parallel 

 furrows J, they contain much less water, than 

 their first view would lead us to suppose ; and 

 the volume of their waters varies so considerably 



* The Thames and the Severn $ and in the New World 

 the Rio Guayaquil, which rises at the foot of Chimborazo, 

 and exhibits a striking disproportion between the brevity of 

 it's course, and the breadth of it's mouth. 



+ For the knowledge of these active sections (sections vives) 

 in the Ganges and the Nile we are indebted to the important 

 labours of Major Renneli and Mr. Girard. 



I See above, p. 464. 



