138 ESSAY 



I. One general fact relative to situation is, 

 that whatever diminishes the view of the sky, 

 as seen from the exposed body, occasions the 

 quantity of dew, which is formed upon it, to be 

 less than would have occurred, if the exposure 

 to the sky had been complete. 



I placed, on several clear and still nights, 10 

 grains of wool upon the middle of a painted 

 board, 4^ feet long, 2 feet wide, and 1 inch 

 thick, elevated 4 feet above the grassplat, by 

 means of 4 slender wooden props of equal 

 height; and, at the same time, attached, loosely, 

 10 grains of wool to the middle of its under- 

 side. The two parcels were consequently only 

 an inch asunder, and were equally exposed to 

 the action of the air. Upon one night, how- 

 ever, I found, that the upper parcel had gained 

 14 grains in weight, but the lower only 4. On 

 a second night, the quantities of moisture, ac- 

 quired by like parcels of wool, in the same 

 situations as in the first experiment, were 19 

 and 6 grains ; on a third, 1 1 and 2 ; on a 

 fourth, 20 and 4 ; the smaller quantity being 

 always that, which was gained by the wool at- 

 tached to the lower side of the board. 



I bent a sheet of pasteboard into the shape of 

 a house-roof, making the angle of flexure 90 

 degrees, and leaving both ends open, This was 

 placed one evening, with its ridge uppermost, 



