Upon a REMARKABLE COLD. 165 



portion of air, by being thrown into a fit of continual refrige- 

 ration, by the action of the furface over which it travels, is,, 

 upon its arrival at the leeward fide, and during the fit, difpofed 

 to part with a greater quantity of hoar-froft in a given time, 

 than what otherwife it would do, even though equally cold. 

 According to this idea, were the furface fo much extended as 

 to bring the fit of refrigeration to a maximum, and fo put an 

 end to it, before the arrival of the air at the leeward fide, a 

 body at this leeward fide would acquire hoar-froft more flowly 

 than another lying fomewhat more to windward, to which the 

 air can reach whilft in the fit of cooling. Thus, therefore, in 

 regard to the greater part of the air, which fweeps the endlefs 

 continuous furface of the fnow upon the ground, we may con- 

 ceive the fit of refrigeration as confummated ; on which ac- 

 count, it will be lefs difpofed than that which crofTes the fand 

 upon the fcale to give out its hoar-froft : And, in this way, the 

 fatfl vmder confideration may be accounted for. 



FROM a defire of making fome more formal experiments 

 upon the air whilft in this progrefs of cooling, by the action, 

 of furfaces expofed to it, I provided a wooden canal, fifteen feet 

 long, a foot wide, and fifteen inches deep. This canal, open 

 at both ends, and with its bottom covered with the fine fand, 

 I intended to have expofed on fome favourable night for the 

 frofty air to glide along it ; and by thermometers placed at cer- 

 tain intervals, as well as plates of metal, &c. which I could 

 bring to the teft of a balance, to have determined the greateft 

 degree of cold, and the rate of cooling, and of giving out hoar- 

 froft, from the windward to the leeward end : But before I 

 could conveniently enter upon this experiment, I was flopped 

 by the thaw. Perhaps a platform of deals laid clofe to one 

 another, and covered, with fand, would be a much. better appa- 

 ratus ; as the more a furface, defigned for fuch a purpofe, is ex- 

 tended in breadth, the warmer air would the lefs readily flow 

 in fidewife, fo as to mar the effecT: produced by the flow cur- 

 rent 



