116 On the Powers of Steam, fcfc. 



These pans are placed in the evening, during the 

 months of December, January, and February ; and 

 in the morning, the water in them is often found a 

 solid piece of ice. Sir Robert does not attribute this 

 to artificial cold, produced by the conversion of water 

 into vapour, but seems to conjecture, that it may 

 arise either from the natural heat of the water escap- 

 ing through the porous vessels*, or else from the 

 cold air finding an easy entrance, through the same 

 pores, into the water. 



With regard to the first supposition, it is reason- 

 able to imagine, that the natural heat of the water 

 would make its escape more readily from the upper 

 surface, where it would meet with no impediment, 

 than through the sides of the vessel ; consequently, 

 that the freezing would take place in any shallow 

 vessel, although it were not so porous as to allow the 

 water to transude. This, however, does not appear 

 to be the case. It must likewise be observed, that as 

 heat always tends to preserve an equilibrium in con- 

 tiguous bodies, it must necessarily follow, that as 

 soon as the temperature of the water had fallen down 



* Whether this is precisely his meaning, with regard to the 

 first supposition, I will not take upon me to determine. I shall 

 transcribe the passage, in his own words, and leave the reader to 

 judge for himself. " The spongy nature of the sugar-canes, or 

 stems of the Indian-corn, appears well calculated to give a pas- 

 sage under the pans to the cold air ; which, acting on the waters 

 or parts of the vessels, may carry oft", by evaporation, a proportion 

 of the heat." As to the second supposition of the cold entering, 

 his words do not admit of a doubt. 



