THE DEW-POINT. 247 



the windows in chambers into which the outer air 

 can always enter freely, or whose atmosphere con- 

 tains no more vapour than does the outer air, 

 cannot become frozen. 



In enclosed buildings, in which much water is 

 evaporated, or which, being continually dwelt in, 

 are guarded from the quick entrance of the air 

 without, even the walls become covered with wet 

 if the cold weather lasts for a long time. You 

 have seen upon such walls the damp which people 

 commonly believe to be a sweat oozing from them. 

 In fact, however, it no more comes out of the walls 

 than that which settles on the window-panes comes 

 out of the glass. In both cases indeed the same 

 cause is at work. You will find that such parts 

 of walls are especially liable to damp as are least 

 open to the warmth of the room ; for instance, 

 those near the floor in the corners, behind the fur- 

 niture, &c. ; and such walls as are most quickly 

 cooled by outward influences ; such, therefore, as 

 have but little thickness, and are built of materials 

 that are good conductors of heat, especially when 

 they are outside walls and exposed to the north. 

 There is no more sure remedy for damp on walls, 

 and for the mouldiness to which it so often gives 

 rise, than a frequent change of the air, which 

 should, if possible, be warm, as it will then be 

 drier. 



When in winter the ground is cooled down 



