388 PHILOSOPHICAL TRANSACTIONS. [annO 1 687. 



of the scales, and exactly counterpoised it with weights in the other scale, and 

 by the application or removal of the pan of coals, we found it very easy to 

 maintain the water in the same degree of heat precisely. Doing thus, we found 

 the weight of the water sensibly to decrease; and at the end of 2 hours we ob- 

 served that there wanted half an ounce troy, all but 7 grains, or 233 grains of 

 water, which in that time had gone off in vapour; though one could hardly 

 perceive it smoke, and the water not sensibly warm. This quantity in so short 

 a time seemed very considerable, being little less than 6 ounces in 24 hours, 

 from so small a surface as a circle of 8 inches diameter. To reduce this expe- 

 riment to an exact calculus, and determine the thickness of the skin of water 

 that had so evaporated, I assume the experiment alleged by Dr. Edward Bernard 

 to have been made in the Oxford society, viz. that the cubic foot, English, of 

 water, weighs exactly yd pounds troy; this divided by 1728, the number of 

 inches in a foot, gives 2534- grains, or ■{ ounce J 34 grains, for the weight of a 

 cubic inch of water; wherefore the weight of 233 grains isf|4 or 35 parts out 

 of 38 of a cubic inch of water. Now the area of the circle, whose diameter 

 is 7^ inches, is 49 square inches; by which dividing the quantity of water 

 evaporated, viz.-f-f- of an inch, the quotient ,44^ or-jV shows, that the thick- 

 ness of the water evaporated was the 53d part of an inch: but we will sup- 

 pose it only the sixtieth part, for the facility of calculation. If therefore water 

 as warm as the air in summer, exhales the thickness of a 6oth part of an inch 

 jn 2 hours from its whole surface, in 12 hours it will exhale the -jij- of an inch; 

 which quantity, will be found abundantly sufficient to serve for all the rains 

 springs, and dews, and account for the Caspian seas being always at a stand, 

 neither wasting nor overflowing; likewise for the current said to set always in 

 - at the Straits of Gibraltar, though those Mediterranean seas receive so many 

 and so considerable rivers. 



To estimate the quantity of water arising in vapour out of the sea, I think I 

 ought to consider it only for the time the sun is up, for that the dews return 

 in the night, as much, if not more, vapours than are then emitted; and in 

 summer the days being longer than 12 hours, this excess is balanced by the 

 weaker action of the sun, especially when rising, before the water be warmed: 

 so that if I allow -yV of an inch of the surface of the sea to be raised per diem 

 in vapours, it may not be an improbable conjecture. 



Upon this supposition, every 10 square inches of the surface of the water 



yields in vapour per diem a cube inch of water; and each square foot half a 



wine pint; every space of 4 feet square, a gallon; a mile square, 6914 tons; 



a square degree, supposed of 69 English miles, will evaporate 33 millions oi 



.tons; and if the Mediterranean be estimated at 40 degrees long and 4 broad. 



