164? &N RAIN, EVAPORATION, &C; 



SECTION 2. 



An Fjlimatc of the Quantity qf Water thatflovs into the Sea from 



England and Watts in a Year. 



Method of de- To calculate the quantity of water that flows down anv one 

 terminmg how . . . .. . _ . ^ 



much water river into the lea in a given time, teems at firlt view a queltion 



flows through a of great difficulty. The necefTary data, however, may be 

 obtained with confiderable exaclnefs, by proper obfervations, 

 and then it becomes an eafy cafe o^menfuration. Dr. Hutton, 

 in his Philos. and Mathemat. Dictionary, article River, pro- 

 pofes a very good method to determine by experiment the ve- 

 locity of a river : — A cylindrical piece of light wood, its length 

 fomewhat lefs than the depth of the waters, is to be taken, 

 and a few fmall weights attached to one end in order to make 

 it fwim upright. To the other end a fmall rod is fixed in the 

 centre in direction of the axis. This being fufFered to float 

 down the dream will move with the velocity of the water ; 

 and if the rod be obferved to incline towards the river upward 

 or downward, it thews the current to be more rapid at the 

 bottom or fur face refpectively. 



This experiment being made in the middle and near the 

 tides of a river, a medium velocity maybe obtained. Then 

 the medium, breadth, depth, and fpace run over in a certain 

 time being multiplied together, will give the quantity of water 

 that flows down in that time. 



Dr. Halley, in order to eftimate the quantity of water that 

 flows into the Mediterranean fea by means of rivers, makes a 

 comparifon of the great rivers of Italy, &c. with that of the 

 JEftimate of wa- Thames. (Philos. TranfaQ. Abridg. Vol. 2. Page 110). He 

 'SeThlmet ^ afruraes the breadth of the Thames at Kmgfton Bridge to be 

 100 yards; its depth three yards, and velocity two miles per 

 hour. He profefledly over-rates the dimenfions, in order to 

 allow more than a fufficiency for the ftreams received below 

 Kingfton. This aflumption gives the area of a tranfverfe fee- 

 tion of the river = 300 fquare yards, and the quantity of 

 water flowing down a± 20.300 000 tons in a day. Thismuft 

 be over-rated by at leafl, I think, one third : — If the breadth 

 be afliimed 100 yards, the depth three, and velocity two miles 

 per hour, it will then give -| of the refult above mentioned ; or 

 it will amount to the fame thing if we take | part from all the. 

 three data aflumed by Dr. Halley, the refult being § of that 



above. 



