RIVER. 



Having the mcaffl or medium velocity of a river, if we 

 multiply this medium, the breadth, depth, and fpace run 

 over in a certain time, the product will give the quantity 

 of water that flows down in that time. Dr. Halley, in 

 order to ellimate the quantity of water that flows into the 

 Mediterranean fea by means of rivers, makes a companion 

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

 (r"hilof. Tranfad. Abridg. vol. ii. page no.) He af- 

 fumes the breadth of the Thames at Kingllon bridge to be 

 loo yards, its depth three yards, and velocity two miles per 

 hour. He profefledly overrates the dimenfions, in order 

 to allow more flian a fufficieiicy for the ftreams received 

 below Kingdom This afl'umption gives the area of a tranf- 

 verl'e fe&ion of the river = 300 fquare yards, and the quan- 

 tity of water flowing down =1 20,300,000 tons in a day. 

 This mud be overrated by at lead, it is fuppoled, one-third. 

 If the breadth be aflumed 100 yards, the depth three, and 

 velocity two miles per hour, it will then give two-thirds of 

 the refult above-mentioned ; or it will amount to the fame 

 thing if we take one-eighth part from all the three data af- 

 fumed by Dr. Halley, the relult being two-thirds of that 

 above; amounting in the year to 166,624, 128,000 cubic 

 feet, which is a little more than one-twenty-fifth part of all 

 the rain and dew in England and Wales in a year, as above 

 deduced. Mr. Dalton has cflimated " that the water of the 

 Thames is drawn from an extent of country of about 600 

 fquare miles, or one-eighth of the area of the whole, nearly. 

 The Severn, including the Wye, fpreads over an equal or 

 greater extent of country ; and that collection of rivers 

 which conititutes the Humber is fuperior to either of the 

 other two in this refpeft. As far as my own obfervation 

 goes, the Severn and Wye mult difembogue as much or 

 more water than the Thames ; the Humber I have not 

 feen collectedly, but have noticed moil of the branches con- 

 ftifuting it, and fliould apprehend it cannot be inferior to 

 the Thames : all other circumftances being the fame, the 

 quantity of water carried down by any river fliould be as 

 the area of the ground from which the water is derived, 

 and on this account the Humber ought to exceed the 

 Thames. 



" The Se'vern, which is partly derived from the moun- 

 tainous country of Wales, is certainly the mod rapid of 

 the three rivers, and probably carries down the mod water ; 

 as the Thames, however, is generally confidered to take 

 the lead, we will fuppofe, upon the whole, that thefe three 

 rivers are equal in this refpect. 



" The counties of Kent, Suflex, Hampfliire, Dorfetfhire, 

 Devonfhire, Cornwall, and Somerfetfliire, from the Med- 

 way to the Lower Avon inclufively, in an extent of 1 1,000 

 fquare miles, do not prefent us with many large rivers. 

 From their number and magnitude, we cannot form a high 

 ettimate of their produce. The quantity of rain for thofe 

 counties is indeed near the average for the kingdom, as far 

 as the preceding obfervations determine ; but the milder 

 temperature of their winters and greater heat of their 

 fprings and fummers, will caulc a greater evaporation than in 

 feme other parts : it is probable the rivers in thefe counties 

 may amount, when taken together, to ii times the mag- 

 nitude of the Thames. The rivers that difembogue their 

 waters on the coaft of Lincolnfhire, Norfolk, Suffolk, and 

 Eflex, from the Humber to the Thames, though drawn 

 from a country of 7000 fquare miles, inanifelUy fall far 

 fhort of the Thames. The two places in this dillrict, for 

 which we have accounts of the rain, Norwich and Up- 

 minller, give a mean of only 22^ inches annually. This, 

 with the flatnefs of the country, which prevents the water 

 from running off ia fome degree, makes the rivers much 



lefs than what might otherwife he expected from the extent 

 of ground. There are but three or four of any confe- 

 quence. Probably all the rivers may amount to half the 

 fi/.e of the Thames. There remains above 6000 fquare miles 

 in Wales, from the Wye to the Dee, inclufivc of the lad, 

 and the northern counties of Lancailer, Weitmoreland, Cum- 

 berland, Northumberland, and Durham, with part of 

 Chefliire, and a fmall part of Yorkfhire, from the Merfey 

 round by the Tweed to the Tees, amounting to 7000 or 

 8000 fquare miles, to be edimated. 



" Thefe two divifions, though not larger than fome others, 

 abound in rivers, many of which are confiderable in mag- 

 nitude and of great rapidity. The rains at an average, it 

 is probable, are double what they are in the S.E. counties 

 of the kingdom. The rivers in thefe two diftri&s cannot 

 fairly be effimated, I think, at lefs than four times the 

 Thames. It appears, then, that by this eflimation, the 

 water carried off by all the rivers in England and Wales, 

 may amount to nine times that carried off by the Thames 

 = 13 inches of rain. There remains itill fixteen times the 

 water of the Thames, or 23 inches of rain to account for, 

 before we have difpofed of all the rain and dew." 



This ingenious philofopher concludes, from a detail of fad* 

 and reafoning, that "the rain and dew of this country are equi- 

 valent to the quantity of water carried off by evaporation 

 and by the rivers. And as nature ads upon general laws, we 

 ought to infer, that it mult be the cafe in every other country, 

 till the contrary is proved. 



" This conclufion being admitted, we are enabled to deduce 

 a general theorem for the quantity of water carried down 

 into the fea by any river in any country (on the fuppofition 

 that all rivers arc ramified alike) provided we have certain 

 data ; thefe data are the length of the river, and the excefs 

 of the rain above the evaporation in the country from which 

 the water of the river is drawn ; alfo, it fhould be known 

 by obfervation, how much water fome one given river carries 

 down. 



" For, from the principles of geometry, the area of country 

 from which any river is fupphed, will be as the fquare of 

 the length of the river ; and the quantity of water carried 

 oft, will be in the compound ratio of the area of the country, 

 and the excels of the rain and dew above the evaporation. 



" Thus, let L = the length of any river, E = the excefs 

 of rain and dew above the evaporation, and Q = the quan- 

 tity of water difembogued in any given time by that river ; 

 / = the length of any other river, e — the excefs, &c. and 



q — the quantity of water; then we (hall have q = -*- — . 



L E 

 "E.gr. Suppofe the length of the Thames = 20c miles, 

 and the excels = 5 inches, eftimating th( rain and dew at 

 30 inches, and evaporation at 25 ; and fuppofe the river 

 Kent, in Weitmoreland, to be 20 miles in length, and the 

 53 inches, the ram and dew being fuppoled 65, and 

 evaporation 30 inches. Then, 



20' x 35 x Q _ tQ _ 



200' x 5 ~ 100 ^' 



or Q — 14; q; which refult, I believe, will be found to 

 accord nearly with the mcafurcment of the two rivers on the 

 principle before-mentioned." Manchefter Memoirs, vol. v 

 pt. 2. 



River, with regard to Agriculture, may be converted, in 

 many lit nation.:, to various ufeful purpofes, Inch as thole 

 of improving the meadow and other grafs-lands which lie 

 below the fources of them, by having their waters in par- 

 ticular feafons turned over them. By proper attention in 



thefe 



