ON PREVENTING THE DUY-ROT IN WOOD. >^l 



^>osed house, and more especially on the most exposed cor- 

 ner of a room seldom warmed by tire, the inner surface of 

 the wall, by the continuance of frost, is become of a very 

 low temperature, like the air within the room itself. So 

 long- as this state of equal temperature between the wall an4 

 internal air continues, or if tlie wall is warmer than that air, 

 it is obvious that the vapour which is mixed with the air can- 

 not part with any heat to the wall, ahd therefore will not un- 

 dergo condensation; just as no dampness appears oa our 

 ■windows during a hot day in summer. But if a thaw comes bm is deposited 



on, and the air becomes warmer than the wall, which, from ^^*^™ ^^^ ^'^ ^ 



. . . . . t"** room, 



its capacity of easily shifting place, it will readily do, then 



the vapour, which is mixed with it, parts with its superabun- 

 dant heat to tlie colder wall, and appears on it in moisture or 

 drops, or pours down it in streams; just as happens to the 

 cold bottle brought into the warm dining-room. 

 ■ This change is the greater, the more completely the ma- 

 terials of the wall fit it for carrying the heat out of the va- 

 pour, or, in philosophical language, the better they conduct 

 heat. Hence a wall painted in oil condenses vapour, or runs a wall painted 

 with water, sooner than one, which, being impainted, is more wiih oil suon- 

 porous; for which reason, in cities, we first perceive damp- ^^^^^^' 

 ness and drops or streamlets of water on tVia oil-painted 

 party walls which bound our staircases, and which are, tliere- 

 fore, absurdly said to sweat, though these walls have no 

 communication with the outward air, and, from their var-^ 

 nished covering, cannot admit of the passage of moisture or 

 perspiration through their pores, 



In this case the remedy is obvious, and by its success Principle of 

 shows the nature of the evil. Prevent your walls from ever pjeveutioa. 

 becoming colder than the warmest external air of winter, and 

 you will never have this appearance of damp on their inner 

 surfaces. 



This may be done, first, by constructing the walls of such Method of ap- 

 a degree of thickness, or with such a disposition or quality plyi'^g ii- 

 of materials, that they shall not, in the usual way, be greatly 

 cooled throughout their whole substance by any temperature 

 of the outward air. With this view, I think that in all single detached 

 houses, which are not warmed by neighbouring fires, and houses require 

 more especiiilly in situations exppsed to high winds, and 1^^*^^^"^ *"* 



therefore 



