i^6 Experiments on Evapdratiw. [Book II. 



hot, and there had been no rain for upwards of a month. 

 When the glafs had flood on the grafs plat one quar- 

 ter of an hour, and had collected a quantity of con- 

 denfed vapour, he wiped its infide with a piece of 

 muflin, the weight of which he had previoufly afcer- 

 tained, and as foon as the glafs was wiped dry the 

 muflin was weighed. The medium increafe of weight 

 from various experiments, between twelve and three 

 o'clock, was fix grains in one quarter of an hour from 

 twenty fquare inches of earth. At this rate of evapo- 

 ration, it is eafy to fee that, computing at feven thou- 

 fand grains troy -to one pint of water, and eight pints 

 to a gallon, not lefs than one thoufand fix hundred 

 gallons of water would be raifed from one acre of 

 ground in twenty-four hours. It may well be fuppofed 

 thar the quantity will be (till greater when the ground 

 has been drenched with rain. In order to prove this, 

 the fame judicious philofopher made two other expe- 

 riments, one of them the day after the ground had 

 been wetted by a thunder-mower; and to afcertain 

 the circumftances more exactly, he took the heat of the 

 earth by a thermometer laid on the grafs, which in the 

 firft experiment was 96', when the evaporation was at 

 the rate of one thoufand nine hundred and ieventy- 

 three gallons from an acre in twelve hours. The other 

 experiment was made when there had been no rain for 

 a week, and when the heat of the earth was 1 10: this 

 experiment gave after the rate of two thoufand eight 

 hundred gallons from an acre in twelve hours; the 

 earth was hotter than the air, being expofed to the re- 

 flexion of the fun's rays from a brick wall *. 



It is the vapour which is exhaled in this manner 

 from the earth, which forms thofe mifts fo commonly 



* Watfon's Chera. E%s, vol. iii. Eff. 2, ' 



obferved 



