274 ESSAY 



and a few smaller vessels, all containing unboiled 

 water. After an exposure of little more than 

 an hour, water in a watch-glass upon the straw 

 was found frozen, the temperature of the air, 

 feet above the straw, being then 37. In 

 half an hour more, ice began to appear in the 

 earthen pans, while a thermometer 5^ feet 

 above them, this being the height at which Mr. 

 Williams used to suspend his instrument, was 

 36. The air soon after became colder; but 

 its temperature was never less than 33, though 

 taken by a naked thermometer, which, as was 

 before said, upon a clear and calm night, 

 occasions the air to seem about 2 colder than 

 it really is. 



It might be inferred, from what is mentioned 

 by Mr. Williams, that the temperature of the 

 straw beds, on which the ice-pans were set at 

 Benares, was always found by him above the 

 freezing point, for this reason, that the straw, 

 from containing no moisture, could not, like 

 the water, grow cold by evaporation. I had, 

 therefore, been surprised, during the first ex- 

 periment, for I had then but little acquaintance 

 with the phenomena of cold observed with dew, 

 that a thermometer, laid upon an exposed part 

 of the straw, was always below the freezing 

 point, after ice had bgun to form in the pans. 

 On reading, however, his account of the process 



