ON RAIN, EVAPORATION, &C. \01 



greateft ; that from green ground is probably next, and that 

 from bare foil the leaft : though we may prefume, that the 

 copious dews upon the grafs more than fupply the excefs of 

 evaporation above what takes place from a moifl uncovered 

 foil. 



The moll fatisfaclory experiments I have feen an account 

 of, relating to the evaporation from a furface of water, are 

 IhofeofDr. Dobfon, made at Liverpool, in the years 1772,Dr. Dobfon's 

 73, 74. and 75. (Vid. Philos. Tranfac. Vol. 67.)— He took a experiments on 



i- i • . ^ , ,- • , .- ii- i evaporation from 



cylindrical veflel or 12 inches diameter, and having nearly water, 



filled it with water, expofed it betides his rain-gage of tho 

 fame aperture, and by adding water to it, or taking it away 

 occaiionally, he kept the furface nearly of the fame height, 

 and carefully regiftered the quantities added or taken away, 

 by a comparison of which with the rain, the amount of the 

 evaporation was afcertained. The mean monthly evapora- 

 tion for four years was — January 1.50 inches. — February 1.77, 

 — March 2.64.— April 3.30.— May 4.34.— June 4.41.— July 

 5.11.— Auguft 5.01.— September 3.18.— O&ober 2.51.-^ 

 November 1.51. — December 1.49. — In all 36.78 inches. The 

 mean rain for the fame time was 37.48 inches. — In the year 

 1793 I found the evaporation from water in a fimilar way at 

 Kendal for 82 days in March, April, May and June to be 

 5.414 inches. The greateft quantity evaporated on one of 

 the hotteft and dried days in fummer was a little above ,2 of 

 an inch in depth. 



The experiments to determine how much is evaporated 

 from green ground and from moifl earth, are very few that 

 have come to my knowledge. Dr. Hales, from a few expe-Hales's eftimate 

 riments, calculates that moifl earth only throws off6f inches fro ™ m0lft 

 annually. — This calculation muft be far below the truth. Dr. 

 Watfon, Bilhop of LlandafF, found that in a dry feafon there j) r . Watfon 

 evaporated from a grafs plat that had been mowed clofe, about {rom a S raf s 

 1600 gallons in an acre per day, which amounts nearly to ,07 p * v 

 of an inch in depth ; and that after rain the evaporation was 

 confiderably more. Now fuppofing ,07 to be the medium 

 daily evaporation for May, June, July and Augufl, and that 

 as much is raifed in thefe four months as in all the reft of the 

 year, the annual evaporation in fuch circumftances will be 17 

 or 1 8 inches, which is but half that obferved from water at 

 Liverpool, and fix inches lefs than the referve of rain flated 

 above. 



In 



