[ 3-^9 ] 



rhere was often no evaporation. Including every variable circumftance, 

 I conclude the hourly evaporation at 32^ may be eftimated at 

 four grains, and at 45*^ at ten grains, at ^o^ at twelve grains, 

 at 55^ at fixteen grains, at 65° at tw-enty grains, and at 75'' at 

 twenty-fix grains, at a: mean throughout the year, in countries 

 where the mean hygrometrical height is LXXXV. and ftorms not 

 very frequent. Hence we may calculate the annual evaporation 

 from maffes of water in countries whofe mean annual tempe- 

 rature and hygrometrical ftate and agitation by tempefls is 

 known. This circumftance, however, has not hitherto been fuf-- 

 ficiently attended to, nor confequently even approximately de^ • 

 termined. 



In London, whofe mean annual temperature is 'about 50*, and 

 where the hygrometer, at a mean of the year, is probably 

 LXXXV. (I fay probably, becaufe its flate in dry and moift years 

 has not been determined) fince the hourly evaporation may be 

 ftated at twelve grains, the daily may be ftated at a mean of the 

 year at two hundred and eighty-eight from a furface of twenty- 

 fix fquare inches, and therefore from one fquare foot one thoufand 

 five hundred and ninety-five grains, which multiplied into three 

 hundred and fixty-five gives 582,175 grains — 83,16716 pounds 

 avoirdupoifc, nearly, or about 10.1,07 pounds troy. Such a quan- 

 tity of water, in a vefi!el of one fquare foot in the bottom and 



R r 2 aperture, • 



