166 EXPERIMENT'S and OBSERVATIONS 



rent advancing to leeward. It would not be wonderful, though 

 fomewhere upon an elevated and detached platform of this 

 kind, we fhould find an artificial cold greater than what pre- 

 vails at the fame time on the fnow at the ground ; as the 

 warmer fnow, which lies immediately beneath, at lead for the 

 mod part, muft more or lefs counteract the cooling procefs kept 

 up there. 



BUT to return from this digreffion, I now come to relate the 

 mofl ftriking experiment I had an opportunity of making this 

 winter, during the time I was occupied in fuch cold vigils. 



IN fome of the fereneft and ftilleft nights, having frequently 

 amufed myfelf by attending to the capricious arrangement of 

 the hoar-frofl as it fettled upon different bodies, I was led to 

 remark, that quickfilver was a long while of attracling any, 

 and even at lafl fparingly, and chiefly at thofe places of its fur- 

 face which were fullied by a dufty film. This naturally fug- 

 gefted a comparifon of quickfilver and the fand, as to their 

 powers of contributing to the cooling procefs at their furface. 



FOR the apparatus, I borrowed from a cabinet-maker two 

 mahogany tea-trays, of an oval form, twenty-two inches by 

 feventeen, and near an inch deep, infide meafure. 



JANUARY 29. 1784. 



ABOUT nine o'clock at night, I placed them out in the park 

 upon a flight plane-tree table, about a foot afunder, and with 

 their longer axes pointing to windward. After levelling one of 

 the trays, its bottom was completely flooded over with quick- 

 filver, to the amount of twenty-four pounds weight, which 

 Meflrs HILL and MONTEATH, furgeons and druggifts here, 

 moft obligingly lent me for that purpofe. The bottom of the 

 other tray was covered to the depth of a quarter of an inch 

 with the fine flinty fand. Upon each of the furfaces of the 

 fand and of the quickfilver, I laid down a thermometer, with 

 the ball near to the leeward end as ufual, and expofed a third 



to 



