150 ESSAY 



of metal, I did not see, that moisture formed 

 in the least more readily, upon the former than 

 upon the latter. I have since learned, that 

 Saussure* once entertained a similar suspicion, 

 which was also proved by an experiment to be 

 groundless. 



All my experiments, hitherto spoken of, were 

 made in the country. But Le Roi having said, 

 that dew is never deposited by the air of cities, 

 I determined to ascertain, if his assertion was 

 just. With this view, I frequently exposed, at 

 night, 10 grains of wool upon a slight wooden 

 frame, placed in such a manner, between two 

 ridges of the top of my house, which is situated 

 in one of the most crowded districts of London, 

 as to be 3 feet distant from the nearest part of 

 the roof. The event was, that, upon clear and 

 calm nights, dew was always acquired by the 

 wool, though never in any considerable quan- 

 tity j probably, however, more from the wooden 

 frame being^ nearly surrounded by buildings, 

 much more elevated than itself, than from any 

 particular condition of the air in cities. The 

 formation of dew, in this situation, proceeded 

 much less regularly than in the country. For, 



* Hygronometrie, page 829. 



