148 ESSAY 



than a similar plate elevated a few inches above 

 the grass, by means of slender props, which 

 allowed the air to pass freely under the metal. 

 But the case with respect to small pieces was 

 the reverse ; for I have often seen, covered 

 with dew, the metallic sheath of a small ther- 

 mometer lying upon grass, while the similar 

 sheath of another thermometer, suspended in 

 the air, remained dry. 



Removing a metal several times, in the course 

 of the night, from one part of the grassplat to 

 another, facilitated its being dewed. The same 

 effect was produced on gilt and silvered paper, 

 by first exposing them to the sky, for some 

 time, with the bare side uppermost, and then 

 turning them. 



If a piece of glass, covered on one side with 

 a metal, be placed upon the ground, with this 

 side downwards, the upper surface will attract 

 dew, precisely as if no metal were attached to 

 the lower surface. 



The upper surfaces of metals are most readily, 

 and most copiously dewed, on those nights, and 

 in those parts of the night, during which other 

 substances are the most readily, and the most 

 copiously dewed. 



If a metallic plate had been laid upon grass, 

 before dew began to form anywhere, its lower 

 side, notwithstanding, always became moist in 



