182 ESSAY 



was, that the sky was not entirely free from 

 clouds; but these were few, of small extent, 

 thin, and high. 



At 6h. 25m. immediately after the sun had 

 ceased to shine upon the spot, where my experi- 

 ments were to be carried on, though the time 

 of its setting was still 47 minutes distant, I 

 placed upon the raised board 1 grains of wool, 

 and a small bag, made of the skin of a swan's 

 breast with the down adhering, and stuffed with 

 wool, the whole weighing nearly 5 drachms. 

 On each of these substances the naked bulb of 

 a small and delicate thermometer was laid. A 

 similar thermometer, with its bulb also naked, 

 was suspended in the air, over the grassplat, at 

 the same height with the board. Two thermo- 

 meters were placed in other situations, as will 

 be seen in the annexed Table. After an ex- 

 posure of 20 minutes, the wool was 7 colder 

 than the air, but the swandown bag only 6, no 

 doubt in consequence of its comparatively great 

 quantity of matter. Neither, however, had 

 gained the least weight, according to the scales 

 employed by me, which were sensibly moved 

 by the 16th of a grain. These observations 

 were repeated several times during the follow- 

 ing hour, as will be seen by the Table, at none 

 of which, except the last, was either the wool 

 or swandown found in the least heavier, than 



