268 Observations on taking down and 



unable to discover truth except by induction, and others costive 

 of their belief in the delirations even of a great teacher, and 

 thinking that they may be desirous of viewing this important 

 question by any glass, however weak its power, your corre- 

 spondent ventures to offer that by which he views the question, 

 and solicits the shelter of a few pages for the following observa- 

 tions in your journal. 



The writers on the ordinances of rivers consider the courses 

 and velocities of them dependent on the nature of the ground 

 over which they pass, as well as upon the heights from which 

 their waters descend. For example, water descending from a 

 height on rocky ground, which it cannot remove, rises, spreads, 

 and forms a lake ; and proceeds with diminished velocity to 

 the lowest point, and there cascades ; advancing at the rate of 

 forty-five inches per second, it will drive flint stones about the 

 size of an egg before it, and rise and spread until its velocity is 

 reduced to thirty-six inches per second, when the stones 

 remain at rest; proceeding among pebbles about an inch dia- 

 meter, it serves them the same, rising and spreading until its 

 velocity is reduced to twenty-four inches per second, when they 

 remain at rest; proceeding through coarse gravel about the 

 size of a marble, it serves it the same, rising and spreading until 

 its velocity is reduced to twelve inches per second ; and so it 

 proceeds with diminished velocity according to the size of the 

 grain, the velocity and the course always varying with the 

 obstacles met with. Gravel, the grain being about the size of 

 aniseed, will be at rest at a velocity of four inches per second ; 

 sand will remain at rest at a velocity of seven inches per second, 

 and precipitate at six inches per second. Clay will remain at rest 

 at a velocity of three inches per second*. By reference to the map 

 of the river Thames west of London Bridge, and bearing the 

 above-mentioned facts in mind, it will appear that the banks 

 of the river from Nine Elms, a little above Vauxhall Bridge, to 

 London Bridge may be considered artificially fenced, and only 

 requiring additional aid by raising and wharfing to prevent over- 



. '^^^ Principes d'Hydraulique, par M. le Chev. Du Buat ; Exphiences stir 

 les Cours des Fleures, par M. Gennetfe ; and the article River, Ency. Brit. 



