i ;6" EXPERIMENTS and OBSERVATIONS 



that more leifurely and intimate communion betwixt the air 



and the fnow, which is necefTary to the extrication of hoar-froft ; 



and that accordingly the thermometer approached confiderably 



nearer to the temperature of the air. 



THAT the extrication of hoar-froft requires an action con- 

 ' tinued for fome length of time, upon a given portion of 



' air," 



is a proposition of fome moment, becaufe, taken along with the 

 other general fact, it enables us fo eafily to explain certain col- 

 lateral phenomena which fo much characterife this kind of 

 cooling procefs j fome examples of which have now been pro- 

 duced. On that account, it might be worth while to eftablifh 

 the propofition farther, by more direct experiments. For this 

 purpofe, a fimple apparatus might be conftracted for making 

 the air, in night-obfervations, to pafs with different rates of ve- 

 locity over the cooling furfaces, in order to difcover how much 

 the extrication of hoar-froft, and the cold produced at the lee- 

 ward fide, depends upon an undifturbed and leifurely commu- 

 nication between the air and the fubftances expofed. As to the 

 iflue, I am already almoft entirely convinced, that, were the 

 natural flow paflage of the air over the furfaces changed fud- 

 denly, by machinery, into a rapid current, the thermometers 

 at the leeward fide would very foon rife, if not to the fame, at 

 leaft nearly to the fame temperature as the air to windward. 



IT might alfo be proper to try how far, and for what length 

 of time, a confiderable quantity of cold air, when {hut up on 

 all fides from the external atmofphere, would, by a leifurely 

 and fucceflive application of all its parts to a furface of fand, 

 &c. keep up a cooling procefs ; and even for this experiment, 

 it would be no difficult matter to contrive a proper apparatus. 



IT is now full time that we relieve the reader, who may have 

 followed us fo far in a difcuffion which has {welled to a much 

 greater extent than originally was intended, and where fo per- 

 petual a recurrence of the fame phrafeolpgy, jiow necefiary fo- 



ever 



