CONGELATION. 345 



mometcr in front of it, which did not descend lower than 

 34®. We have here an instance of congelation where 

 hoar frost could not exist; but suspecting evaporation to 

 be the cause, I had the thermometer dipped in water of 

 36°, and exposed it again to the breeze; upon which the 

 mercury fell to 29.5°, and a film of ice formed on the 

 bulb in a shprt time. Two small parcels of wet yarn 

 were also placed in the same situation, one of them being 

 inclosed in a corked phial, while the other remained ex- 

 posed to the wind ; the latter soon became stiff, but the 

 former continued soft for the space of three hours. 



The experiments detailed in this letter have convinced The theory of 

 me, and the same evidence perhaps may convince some M. Dispan not 

 of your readers, that M. Dispan's theory cannot be re- upon, 

 lied on ; for water has been shewn to congeal without the 

 assistance of hoar frost, when the freezing powers of the 

 atmosphere are very feeble ; which appears to be an un^ 

 answerable objection to the theory in question. As for 

 the phenomenon described by M. Dispan, perhaps it is 

 not very uncommon; at least I have known something 

 similar occur more than once on a part of the river at 

 Kendal, where the current is obstructed by a weir placed 

 a little below a stone bridge. The expanse of water 

 formed by this impediment is occasionally covered with a 

 sheet of thin ice above and below the bridge in the course ' 

 of a night of calm and moderate frost, when the river is 

 low and nearly stagnant; at the -^ame time no symptoms 

 of congelation are seen under the arches. I have always 

 attributed this singular occurrence to the same cause 

 which preserved my water pots on the window sole from 

 the frost ; that is, I have always supposed that the supe- 

 rior temperature of the bridge prevented the two sheets 

 of ice from uniting beneath it, nor could conjecture fur- 

 nish me with another reason. M. Dispan^s memoir, 

 however, suggested new principles ; and as there was a 

 possibility of error on my part, I have endeavoured to 

 throw fresh light on the subject by the experiments 

 which are now submitted to your consideration. 



I remain, &c. " 



JOHN GOUGH. 



