826 On Preventing the Decay of Wood. 
apply. But it is not possible to prevent some exchange 
of this kind through the atmosphere of any habitable 
chamber; and it is evident from the moisture being most 
abundantly, or perhaps solely, deposited on the inside of 
that part of the wall which is most exposed to the exter- 
nal cold, that the chief or common mode in which the 
Wall is cooled is not by the access of the cold air into the 
room, but by the passage of heat from the wall itself into 
the cold air without. We may however so far avail our- 
selves of this principle, as to exclude as much cold air ns 
we can, by shutting up the windows and chimnies of un- 
inhabited rooms during the severity of frost. 
It may farther be suggested, that as, during a thaw, the 
air, being warmer than in frost, has a greater quantity of 
water in form of vapour mixed with it, shutting up a room 
on such occasions may, by retarding the admission of 
warmer air so charged with vapour, allow time for the 
walls to acquire an equable temperature through their 
substance from without, so as to anticipate any condensa-* * 
tion on their surface which might occur from the free 
admission of the external air. To this I only answer, 
as before, that rooms according to the common construc- 
tion cannot be excluded from communication with the ex- 
ternal air; and that, in fact, the dampness does under 
these circumstances take place, though the doors and win- 
dows are never opened. 
In all cases, however, there is one method of prevent- 
ing this species of dampness, which is infallible; and 
that is to keep every part of the internal surface of the 
wall in the chamber or staircase sufficiently warm by 
good fires. With this view all staircases ought to have 
some means of receiving artificial warmth. 
If, notwithstanding this and the former precaution, a 
wall should accidentally become damp, the next best ex- 
