268 ESSAY 



degree of cold, which might be produced by 

 evaporation from water contained in a shallow 

 vessel. With this view, I placed on a feather- 

 bed, situated between the door and window of 

 a room in my house in London, two china plates, 

 into one of which as much water was poured, 

 as covered its bottom to the depth of ^ of an 

 inch. The other plate was kept dry. The 

 bulb of a small thermometer being then applied 

 to the inside of the bottom of each plate, I ob- 

 served upon many days, in various seasons of 

 the year, the difference between these instru- 

 ments while the door and window were open. 

 I found, in consequence, that when the tem- 

 perature of the air in the room was 75, the 

 highest at which any experiment was made, the 

 thermometer in the plate, containing water, was 

 between 6 and 7 degrees lower than the one in 

 the dry plate ; that the difference between these 

 thermometers diminished gradually as the air 

 became colder ; and that when the temperature 

 of the air was 40, the lowest for which I have 

 any observation, the difference was only lj. 

 At 32, therefore, it would have been very 

 small, and at a few degrees below 32 it would 

 probably have vanished. This supposition agrees 

 with an observation made by Mr. Wilson of 

 Glasgow, who found, that no cold was pro- 

 duced by evaporation from snow possessing a 



