824 On Preventing the Decay of Wood . 
Becomes warmer than the wall, which, from its capacity 
of easily shifting place, it will readily do, then the va- 
lour, which is mixed with it, parts with its superabun 
dant heat to the colder wall, and appears on it in mois- 
ture or drops, or pours down it in streams ; just as hap- 
pens to the cold bottle brought into the warm dining- 
room* 
This change is the greater, the more completely the 
materials of the wall fit it for carrying the heat out of the 
vapour, or, in philosophical language, the better they 
conduct heat. Hence a wall painted in oil condenses va- 
pour, or runs with water, sooner than one, which, being 
nnpainted, is more porous ; for which reason, in cities, we 
first perceive dampness and drops or streamlets of water 
on the oil-painted party walls w hich bound our stair- 
cases, and which are, therefore, absurdly said to sweat, 
though these walls have no communication with the out- 
ward air, and, from their varnished covering, cannot ad- 
mit of the passage of moisture or perspiration through 
their pores. 
In this case the remedy is obvious, and by its success 
shows the nature of the evil. Prevent your walls from 
ever becoming colder than the warmest external air of 
winter, and you will neyer have this appearance of damp 
on their inner surfaces. 
This may be done, first, by constructing the walls of 
such a degree of thickness, or with such a disposition or 
quality 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 hours, which are not warmed by 
neighbouring fires, and more especially in situations ex- 
posed to high winds, and therefore to great evaporation 
from the external surface, and consequent abstraction of 
heat, the walls should always be double, having on the 
