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The practice of all ventilation is founded on the 

 principle that the hottest air rises to the highest 

 part of the house, and if there allowed to escape, 

 colder air will come in below to supply its place. To 

 prevent the hot air escaping too rapidly, the ven- 

 tilators should be fitted with doors or caps, capable 

 of regulating the size of the orifice ; and the openings 

 admitting fresh and colder from without, should have 

 similar regulators, and be made by means of pipes 

 passing through the bark-bed, tank of hot water, or 

 other source of heat, so that the reduction of tem- 

 perature be not too rapid. 



Some guide in constructing ventilators proportioned 

 to the size of the house to be ventilated, will be found 

 in Mr. Hood's following table of the quantity of air, 

 in cubic feet, discharged per minute through a ven- 

 tilator, of which the area is one square foot. 



