ESSAY ON DEW, &c. 



INTRODUCTION. 



I WAS led, in the autumn of 1784, by the event 

 of a rude experiment, to think it probable, that 

 the formation of dew is attended with the pro- 

 duction of cold. In 1788, a paper on hoarfrost, 

 by Mr. Patrick Wilson of Glasgow, was pub- 

 lished in the first volume of the Transactions 

 of the Royal Society of Edinburgh, by which 

 it appeared, that this opinion had been enter- 

 tained by that gentleman, before it had oc- 

 curred to myself. In the course of the same 

 year, Mr. Six of Canterbury mentioned in a 

 paper communicated to the Royal Society, that, 

 on clear and dewy nights, he always found the 

 mercury lower in a thermometer laid upon the 

 ground, in a meadow in his neighbourhood, 

 than it was in a similar thermometer suspended 

 in the air, six feet above the former ; and that, 

 upon one night, the difference amounted to 5 

 of Fahrenheit's scale. Mr. Six, however, did 



