r^f, ON PREVENTING THE DRY-ROT IN WOOD. 



therefore to great evaporation from the external surface, and 

 consequent abstraction of heat, the walls should always be 

 double, having on the inside a thin layer of brick, with an 

 interval of one or two inches frona the outer and thicker 

 layer of brick or stone, to which it must be united by proper 

 binders. The porous structure of the bricks, added to the 

 impermeableness pf the intermediate stratum of air, would 

 so ill conduct heat, that such walls \yould necessarily tend 

 to keep a house dry and warm in tlje winter, as well as cool 

 in the summer. This end would be still further promote4 

 by filling the interval between the two layers with dry sand. 

 Hew the com- fresh sifted coal-ashes, or powdered charcoal. In fact, when 

 pica methods ^^^^ common external means before described have succeeded 

 ceed. in curing dampness, it has been either by aftording a varnish, 



which has diminished evaporation by preventing absorption, 

 or by increasing the space or changing the quality of the 

 ijiaterials of the wall through which the heat was to pass, sp 

 as in either of these cases to retain it more forcibly : And 

 I when the dampness has been remedied by removing the pa- 



per to some distance from the wall by rneans of strained 

 canvas, that eifect has been produced by rendering the pa- 

 per a worse conductor of heat; and therefore indisposing it 

 to condense the vapour in the room so readily as when it 

 jvas in contact with the colder wall. 

 Notpossibleto It has been suggested, that it would be possible to keqp 

 keep out the put cold, or, in more apcurate language, prevent the egre^§ 

 of heat from the inside of a room, and therefoVe frontj the 

 walls surrounding it, by shutting it closely up, and preventf 

 ing any admission pf the cold external air. This has arisen 

 from the supposition that air is not a gopd conductor or 

 transmitter of heat through its substance or pores, but that 

 it merely carries it by changing place with some other por- 

 tion which vvas less charged with it. If there were no other 

 mode of abstracting the heat from the walls of a room, arjd 

 if it were possible wholly to prevent any change of its air. 

 Not wholly, this theory might perhaps apply. But jt is not possible to 

 pfevent some exchange of this kind through the atmosphere 

 of any habitable chamber; and it is evident from the mois- 

 ture being most abundantly, or perhaps solely, deposited pn 

 fchf inside of that part of the wall whigh is most exposed tQ 



