Upon a REMARKABLE COLD. 175 



BUT when the air is in a contrary (late, and difpofed to give 

 out hoar-froft, the ball, with the fnow round it, may be con- . 

 fidered, according to the new ideas we have acquired, as not 

 affording furface enough for the extrication of hoar-froft from 

 any portion of air, during fo momentary a contact, in paffing 

 to leeward. For, from feveral phenomena already infifled upon, 

 it feems evident, that an action, continued for fome length of 

 time, is neceffary for unlocking the hoar-froft from the air, and 

 conveying it to the attracting furface. Hence the thermometer 

 with the fnow round the ball, in the circumftances fuppofed, 

 fhould not be cooled more than the current of air in which it 

 is placed ; and thus this characleriftical diftinction. betwixt the 

 two' procefles may be accounted for. 



I N like manner, may be underftood, how it is in the clearefl 

 and ftilleft nights only, that the cold at the furface of the boards 

 was obferved to be moil remarkable. For, according to what 

 has been juft now mentioned, any very rapid motion of the 

 air acrofs thefe furfaces, feems inconfiftent with the extrication 

 of hoar-froft, upon which the refrigeration depends. 



FURTHER, in the paper of the year 1780, an account is given 

 of an experiment, which confifted in blowing the ambient cold 

 air upon the fnow where the thermometer lay, and of fanning 

 the fame brLfltly with a piece of ftiff paper faftened to the end 

 of a long ftick, in expectation that the thermometer would 

 point lower by an increafed evaporation. This experiment 

 was made pretty early, and whilft I was in the perfuailon, that 

 the obferved excefs of cold depended entirely upon evaporation. 

 The refult, however, was, that the thermometer, inftead of 

 pointing lower by fuch means, always rofe feveral degrees. 

 Though this circumftance was thought odd and unaccountable 

 at the time, yet the true reafon of it feems now to offer itfelf 

 very plainly. For, from the explanations attempted in the two 

 foregoing paragraphs, it would appear, that, by the operations 

 with the bellows and fan, we had all the while been difturbing 



that 



