and the Economy of Fuel. 47 



from passing out of it, one would imagine it could re- 

 quire no great effort of genius to discover that it would 

 be equally efficacious for preventing the heat without 

 from coming in. But natural as this conclusion may 

 appear, I believe it has never yet occurred to anybody ; 

 at least I am quite certain that I have never seen a 

 double window either in Italy or in any other hot 

 country I have had occasion to visit.* 



But the utility of double windows and double walls, 

 in hot as well as in cold countries, is a matter of so 

 much importance that I shall take occasion to treat it 

 more fully in another place. In the mean time, I shall 

 only observe here that it is the confined air shut up 

 between the two windows, and not the double glass 

 plates, that renders the passage of heat through them so 

 difficult. Were it owing to the increased thickness of 

 the glass, a single pane of glass twice as thick would 

 answer the same purpose ; but the increased thickness 

 of the glass of which a window is formed is not 

 found to have any sensible effect in rendering a room 

 warmer. 



But air is not only a non-conductor of heat, but its 

 non-conducting power may be greatly increased. To 

 be able to form a just idea of the manner in which air 

 may be rendered a worse conductor of heat, or, which 

 is the same thing, a better non-conductor of it than it 

 is in its natural unconfined state, it will be necessary to 

 consider the manner in which heat passes through air. 



* When double windows are used in hot countries to keep dwelling-houses 

 cool, great care must be taken to screen those windows from the sun's direct 

 rays, and even from the strong light of day, otherwise they will produce effects 

 directly contrary to those intended. This may easily be done either by Vene- 

 tian blinds or by awnings. In all cases where rooms are to be kept cool in hot 

 weather, the less light that is permitted to enter them the cooler they will be. 



