PRINCIPLES OF VENTILATION. 261 



temperature of our houses by artificial means, and drive oiT the 

 oxygen and aqueous vapor, without returning a supply. We 

 admit the heat to escape through the laps and fissures of the 

 glass, of which there is always enough in badly glassed houses 

 to admit the escape of one fourth the heat radiated in the house. 

 And, moreover, we allow one fourth more at least to be taken 

 away by direct radiation from the glass, so that hardly one half 

 of the heat generated is used for the purpose intended. And, 

 lastly, we admit the external air into the house, to deprive the 

 atmosphere of its moisture by condensation. Likewise, in sum- 

 mer, we admit the external air, in Sirocco currents, to sweep 

 through the house, carrying away the moisture daily by gal- 

 lons; and which, if not returned in equal abundance, must 

 speedily prove injurious to the plants. 



