Upon a REMARKABLE COLD. 



167 



to the air, about a foot to windward, and even with the trays. 

 I then left them for an hour to cool ; after which I continued 

 to obferve them now and then for two hours more. During all 

 this time, I found, that the thermometer placed upon the 

 quickfilver continually pointed higher than the other upon the 

 fand, as fet down in the following regifter : 



To make fure of every thing in this experiment, especially 

 as the extremes of temperature were but inconfiderable, I fome- 

 times changed the thermometers, and fhifted the fand-tray 

 from the right to the left fide of the other, without at all alter- 

 ing the report. Whilfl this experiment was going forward, I 

 difcerned plainly all the common fymptoms of the hoar-froft 

 joining the fand, and fettling upon the infide and outfide wood 

 of both trays. Though at length, fome made its appearance 

 upon the face of the quickfilver, yet I am apt to think, that 

 the cold generated all around by the hoar-froft which attached 

 itfelf to the wood, was the chief caufe of the quickfilver keep- 

 ing below the temperature of the air to windward. 



IT afterwards occurred to me, that, in this experiment, it 

 would have been proper to have filled the tray with quickfilver 

 to the very brim, and to have defended the outfide wood from 

 the air by fome rare fubftance. Had things been fo managed, 

 and the other tray completely filled alfo with fand, and defended 

 in the fame manner, I am perfuaded the refult would have been 

 more remarkable ; and that the qxiickfilver, thus expofed with 



an 



