1 64 EXPERIMENTS and OBSERVATIONS 



perature as the air, the fcale gradually became lighter, though 

 in a fmall degree. 



REMARKS. 



IN eftimating the degree of refrigeration at the furfaces of 

 the various fubftances upon the board, the thermometers were 

 always laid down at the leeward fide, as has been fo frequently 

 mentioned. This was foon found to be a neceflary precaution ; 

 for, when expofed in any other fituation, the cold mown was 

 lefs confiderable. I was, indeed, led very early to fufpect fuch 

 a refult, upon confidering, that this very cold air, though 

 prone to give out hoar-froft, might not all at once be difpofed 

 to part with what was redundant, though folicited by bodies 

 capable of receiving it. The confequence of this feemed to be, 

 that any given portion of this very cold air, in flitting flowly 

 over the fand, ?fff . would part with more and more hoar-froft, 

 and, on that account, become colder and colder, fo as to be 

 coldeft of all near the leeward fide ; or, in other words, would 

 be thrown into a Jit of continual refrigeration in its paflage over 

 the boards, which circumftance was found to be agreeable to. 

 experience. 



THIS fact, therefore, feems very confonant to all the reft, as 

 indicating, in another way, the relation between the reparation 

 of hoar-froft from the air, and the apparent wafte of fenfible 

 heat. Having brought this topic into view, it may perhaps be 

 worth while to purfue it, for a moment, a little farther. 



IT has been already obferved, that the thermometer upon the 

 fand-fcale was much fooner crufted over with, hoar-froft than 

 the one in air to windward. This, indeed, is what might have 

 been expected, on account of the laft of thefe two fituations be- 

 ing the warmeft. But as the thermometer upon the fnow at 

 the ground was always colder, than that upon the fand, it does 

 not fo readily appear, how this laft mould have attracted hoar- 

 froft fooner than the firft. This fact feems to fhow, that any 



portion 



