160 ESSAY 



two other nights, the difference between two 

 thermometers in the same situations was 8* I 

 found also, on two other serene and calm nights, 

 a spot of grass covered by the pasteboard roof, 

 and another spot surrounded by the earthen 

 cylinder, to be both 10 warmer than neigh- 

 bouring grass fully exposed to the sky. Think- 

 ing it possible, that the cylinder, which had 

 been exposed to the sun the preceding day, 

 might still possess some of the heat, which it 

 had then imbibed, I placed near to it, on an- 

 other night, a cylinder made of very thin paste- 

 board ; but this was equally efficacious with the 

 earthen one, in preventing cold from occurring 

 on grass. When the exposure was greater than 

 in the preceding examples, and more dew was 

 in consequence formed, the cold was also 

 greater, but still less than where the exposure 

 was complete. For instance, upon the night 

 during which 10 grains of wool, placed upon 

 the middle of the grass, which was sheltered 

 by the raised board, had gained 7 grains, and 

 the same quantity on grass fully exposed to the 

 sky had gained 10 grains, the difference be- 

 tween the temperatures of the two portions of 

 grass was only 2j. 



The same correspondence was observed, when 

 the differences in the quantity of dew did not 

 depend, as in the preceding instances, upon 



