162 OX RAIN, EVAPORATION 1 , &e. 



much as the inland counties of England, not in the above liff, 

 would fall fhort ; becaufe Wales is both a mountainous coun- 

 try, and exppfed to the fea. 



'mches^rEn- We WiI1 t ' iereforc oqnc^tt^c^l that the mean annual depth of 

 land and Wales. ram m England and Wales, deduced from thefe 20 counties, 

 is 31 inches : A quantity which fubfequent obfervations, I ana 

 confident, will not diminifli, and probably not increafe much *. 

 Quantity of dew. it remains to eftimate the quantity of dew that falk in a 

 fited bjrthe cold vear - Some have doubted whether dew is derived from the 

 of night. air or the earth ; but a proper attention to the phenomena will 



fatisfy us, that it is a depolition of water, evaporated during 

 the heat of the day. With refpecl: to the quantity that falls in 

 a year, we are much at a Iofs, as no daily obfervations have 

 been made for a feries of time that I know of: indeed, it 

 would be difficult to prefcribe a mode of obfervation. Dr. 

 Eftimate by Dr. Hales f relates fome experiments made to determine the quan- 

 tity of dew that falls upon moift earth, from which he eftimates 

 the annual dew at 3. 28 inches. But it is probable that tha 

 dew which is depofited on grafs is much more copious than 

 \ what falls on moift earth, becaufe grafs expofes much more 

 Annual quantity furface in a given acre of ground. If we take the dew at five 

 five Inches!" ' mcnes annually, it will probably not be much over-rated : 

 fuppofing it mould be over-rated, the excels may fiand againft 

 the rain that is loft by evaporation from the furface of the rain- 

 gage each time it rains j. W T herefore, upon the whole, we 



fhall 



* The editors of the Encyclopedia, under the article Weather, 

 from 16 places of obfervation^ make the annual mean for Great 

 Britain 32. 53 inches j and M. Cotte, in the Journal de Phyfique 

 tor 1791, gives a mean derived from 147 places indifferent parts of 

 the world equal to 34. 7 inches, 

 t Veg. Statics, Vol. I. page 52. 



X Since writing the above paragraph on dew, I have had o.cca« 

 flon to make feveral experiments on the fubje6t of aqueous vapour, 

 as it exifts in the atmofphere, the refult of which will, I am per- 

 fuaded, materially illuftrate this important queftion in phyfics.— At 

 prefent I mail only ohferve, that the following conclufions feem 

 deducible from the experiments above referred to. 

 Generalities V' That aqueous vapour is an elaftic fluid fui generis > diffufible- 



concerning aque- in the atmofphere, but forming no chemical combination with it. 

 cus vapor. 2i That temperature alone limits the maximum of vapour in th* 



atmofphere. 



3. That 



