184 ESSAY 



Similar experiments made at the same place, 

 on the evenings of the 25th of August and 17th 

 of September, in the same year, had results, 

 which were also similar but less in degree ; the 

 greatest difference between the temperature of 

 wool or swandown, while they were without any 

 increase of weight, and the temperature ^e 

 air, having been, on the first of those even .,s, 

 only 4, and on the second only 5. The rea- 

 sons were, in great measure, if not wholly, that 

 a considerable part of the sky was covered with 

 clouds, and that the air was commonly in that 

 state of motion, which is denominated a gentle 

 breeze. 



On the evening of my first experiments, I had 

 omitted to measure the heat of the raised board, 

 before the thermometers were placed upon it. 

 This was attended to on the two latter evenings, 

 on the first of which its upper surface was found, 

 at the commencement of the experiments, 4 

 warmer than the air ; on the second, both it and 

 the air were of the same temperature. Again ; 

 on the first of the latter evenings, 10 grains of 

 wool, to which 3 grains of water had been added, 

 having been laid on the raised board, near the 

 thermometers ; at the end of 45 minutes the 

 parcel was found to have lost 2^ grains of mois- 

 ture by evaporation, during the time, that dry 

 wool had become several degrees colder than 

 the air. 



