1 3O Springs , Rivers, and the Sea. 



The sand thus raised is deposited in pla- 

 ces where the water slacks its velocity, 

 and there by degrees an obstacle, a bank, 

 and even an island, is formed, which in 

 its turn produces other changes. Thus a 

 river sometimes forms itself a new bed, 

 or it oversows the adjacent grounds. k 



In some places we find that an obstacle 

 or a bent on one side will occasion a cor- 

 rosion on the opposite bank, by directing 

 the impetus of the stream towards that 

 bank. Thus, from divers causes, whose 

 concurrence in different proportions, and 

 at different times, forms an infinite varie- 

 ty, the velocity of rivers is never steady 

 or uniform. 



The following curious calculation res- 

 pecting the river Thames was made by Dr 

 Halley. " In order to estimate the quanti- 

 ty of water, which passes daily through 

 the Thames, the Doctor assumes the 

 breadth of the river at Kingston bridge, 

 (where the flood seldom reaches) to be 

 100 yards, and the depth 3 ; so that the 

 section of the channel is 300 square yards, 

 and allowing the velocity of the \vater to 

 be at the rate of 2 miles per hour, there 

 will run in 24 hours, the length of 48 miles 



