ON RAIN, EVAPORATION, &C. 165 



above, amounting in the year to 166.62-kl28.00Q cubic ket, 



which is a little more than ~ T part of all the rain and dew in 



England and Wales in a year, as above deduced, 



By an infpeaion of the annexed map of the rivers of this View of all the 



. , , ,i rivers or -trig- 



country, as well as by a fair calculation, it appears, that the lan(1 and Walcs> 



water of the Thames is drawn from an extent of country of 

 about <500 fquare miles, or ~ 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 

 constitutes the Humber is fuperior to either of the other two 

 in this refpecl. As far as my own obfervation goes, the Se- 

 vern and Wye rauit difembogue as much or more water than 

 the Thames ; the Humber 1 have not (een colle&edly, but 

 have noticed moll of the branches conftituting it, and fliould 

 apprehend it cannot be inferior to the Thames : all other cir- 

 cumltances 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 Severn, which is partly derived from the mountainous and the dlftri&s , 

 country of Wales, is certainly the moil rapid of the three ri- 

 vers, and probably carries down the moll 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 refpett. 



The counties of Kent, Suflex, Hampfliire, Dorfetfliire, which fupply 

 Devonfhire, Cornwall, and Somerfetftiire, from the Med way * hem with w ^ ? * 

 to the lower Avon inclufively, in an extent of 11.000 fquare 

 miles, do not prefent us with many large rivers. From their 

 nnmber and magnitude, we cannot form a high eftimate of 

 their produce. The quantity of rain for thofe counties is in- 

 deed near the average for the kingdom, as far as the preced- 

 ing obfervations determine ; but the milder temperature of 

 their winters and greater heat of their fp rings and fummers, 

 will caufe a greater evaporation than in fome other parts : It 

 is probable the rivers in thefe counties may amount, when 

 taken together, to l-§ times the magnitude of the Thames. 

 The rivers that difembogue their waters on the coaft of Lin- 



* A more per feci theorem will be given afterwards, for finding 

 tfie quantity of water carded down by any river* 



colnfiiire, 



