CONGELATION. 343 



that congelation is caused in Spring and Autumn by hoar 

 frost falling perpendicularly from the atmosphere; but 

 the morning ofthe22rid exhibited appearances which re- 

 concile the apparent contradictions of my experiment on 

 very diflercnt principles. The roads were dirty, in con- 

 sequence of a week of very tempestuous weather, and the 

 surface of the mire upon them was slightly frozen; but 

 the small collections of water and mud which were found 

 near the house remained untouched by the frost, even in 

 situations where the eves of the building could not pro- 

 tect them from the perpendicular descent of particles 

 falling from the atmosphere. The foregoing observation 

 induces me to conclude, that the superior temperature of 

 the house counteracted the feeble effects of the frost to a 

 certain distance from itself, and raised the temperature of 

 a vertical stratum of air above the freezing point : now as 

 the vessel stood on the sole of the window, on which my 

 observations were made, it was wholly surrounded by 

 this warm stratum ; consequently, the water contained 

 in it could not freeze, while the saucer in the middle of 

 the garden was exposed to a degree of temperature lower 

 thanSS''. 



I am farther convinced of the justice of this conclu- Houses and 

 sion by having frequently found that houses and strong walls do not 

 walls do not heat and cool with the same celerity as the ^g quickly aa 

 atmosphere. This appears to be the reason why slight the atmos^ 

 frosts are found to prevail in Spring and Autumn in the P"^*^^* 

 open fields, which do not extend their effects to towns ; 

 while, on the contrary, exposed roads are frequently 

 observed to be wet and dirty after the conclusion of a fit 

 of severe weather, at a time when the ground close to high 

 buildings remains frozen. To prove that this Avas the 

 real cause of the contradiction apparent in my experi- 

 ment, I repeated it on a subsequent occasion with this 

 addition: two Florence flasks were exposed to the air, 

 one of them being suspended on a tree in the middle of 

 the garden, and the other on the branch of a cherry tree 

 near a pot of water standing in the window mentioned 

 before. The flask in the middle of the garden was soon 

 incrusted with rime, and a film of ice had been formed in 

 the mean time upon a plate of water standing in the open 

 AAa 2 air; 



