On Preventing the Decay of Wood. 327 
pedient is to dry it as quickly as possible by a free cur- 
rent of warm air. 
This discussion, which at first sight might appear te- 
dious and irrelevant, will, I trust, no longer be thought 
so, when it shall have been found necessary for the esta- 
blishment of a principle on the subject more immediately 
before us. 
In order to show the analogy, let us take the simplest 
example, which is that of a wainscotted room, unw armed 
by fires. When the wainscot is colder than the air, it 
condenses the vapour in form of moisture. If that mois- 
ture were exposed to the influence of the sun and wind*, 
the case would come under the former head of decay, 
which is that of wood wetted by rain in the open air. 
The water soon evaporates, and little decay proceeds in 
the wood. So in the wainscot, the surface next the 
room, though unprotected by paint, will perhaps be long 
in rotting, because the room admits of currents of air, 
more especially when doors and windows are frequently 
opened, so as to evaporate the superficial moisture, though 
less quickly and effectually than in the open air. But 
what is the case with the surface of the pannel next the 
wall? The air, loaded with moisture, penetrates into 
that interstitial space, and deposits it by condensation on 
that surface. But there is afterward no current of air to 
evaporate the water so deposited, which then slowly de- 
composes and destroys that surface of the pannel. Such 
is precisely the process of the dry rot, which always be- 
gins next the wall, and gradually proceeds to the painted 
or outer surface of the wood. It resembles in its chief 
circumstances the decay of paper in a damp room ; and it 
precisely resembles that of paper projecting from the wall 
on canvass, which will still often happen, if the wail be 
subject to acquire a very considerable degree of coldness, 
though much more slowly than in the former case. 
