140 ESSAY 



substances, under which the wool was placed, 

 prevented, mechanically, the access of that fluid. 

 But on this supposition it cannot be explained, 

 why some dew was always found in the most 

 sheltered places, and why a considerable quan* 

 tity occurred upon the grass under the middle 

 of the raised board. A still stronger proof of the 

 want of justness in this supposition is afforded 

 by the following experiment. I placed, up- 

 right, on the grassplat a hollow cylinder of 

 baked clay, the height of which was 2^ feet, 

 and diameter 1 foot. On the grass, surrounded 

 by the cylinder, were laid 10 grains of wool, 

 which, in this situation, as there was not the 

 least wind, would have received as much rain, 

 as a like quantity of wool fully exposed to the 

 sky. But the quantity of moisture, obtained 

 by the wool surrounded by the cylinder, was 

 only a little more than 2 grains, while that ac- 

 quired by 10 grains of fully exposed wool was 

 16. This occurred on the night, during which 

 the wool under the bent pasteboard gained only 

 2 grains of moisture. 



Dew, however, will, in consequence of other 

 varieties of situation, form in very different 

 quantities, upon substances of the same kind, 

 although these should be similarly exposed to 

 the sky. 



In the first place ; it is requisite, for the most 



