We took a Pan of Water, about 4 inches deep, and 7 inch- 
es 5^ diameter, in which we placed a Thermometer, and by 
means of a Pan of Coals, we brought the Water to the fame 
degree of heat which is obferved to be that of the Air in our 
hocteft Summers 5 the Thermometer nicely fhewing it. This • 
done, we affiled the Pan of Water, with the Thermometer in 
it, to one end of the Beam of the5caies, and exadtly counter- 
poifed it with weights in the other Scale ; and by the applica- 
tion or removal of the Pan of Coals, we found it very eafie 
to maintain the Water in the fame degree of Heat precifely. 
Doing thus, we found the weight of the Water fenfibly to de- 
creafe; and at the end of tvvo hours we obferved that there 
wanted hali an ounce Troy^ all but 7 grains, or 3 grains of 
Water, which in that time had gone off in Vapour ; tho one 
could hardly perceive it fmoak, and the Water were not fen- 
fibly warm. This Quantity in fo ftiort a time feemed very 
confiderable, being Hccle lefs than 6 ounces in 24 hours from 
fo fmall a Surface as a Circle of 8 Inches diameter. To reduce 
this Experiment to an exaft Calculus and determine the thick- 
nefs of the skin of Water that had fo evaporated, laffiime the 
Experiment alledged by Dr. Edward Bernard to have been 
made in the Oxford Society, viz. That the Cube foot, Englifb, 
ot Water weighs exaftly 76 pounds Troy ; this divided by 
1718, the number of inches in a foot will give 25 jf grains, or 
I- ounce 131 grains for the weight of a Cube inch of Water; 
wherefore the Weight of 233 grains isf-f^ or 35* parts of 38 of 
a Cube inch of Water. Now the Area of the Circle, whofe Di- 
ameter is 7 n. inches, is 49 fquare inches ; by which dividing 
the quantity of Water evaporated, viz. 44 of an inch, the Quote 
rl^ or Ihews that the thicknefs of the Water evaporated, 
was the 5 3^/ part of an Inch: but we will fuppofe it only the 
fixcicth partj for the facility of Calculation. If therefore Wa- 
ter as warm as the Air in Summer, exhales the thicknefs of a 
60 part of an inch in two hours from its whole Surface, in 
twelve hours it will exhale the ^ of an inch; which quantity, 
Will be found abundantly fufiicient to ferve for all the Rains, 
Springs 
