358 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK IT. 



time the heat is most rapidly lost. An excellent method of securing 

 oneself against this loss is the employment of double windows ; the 

 air which is imprisoned between the two slender partitions of glass is 

 not renewed, and being a badly conducting substance, forms a protec- 

 tive covering, which the obscure heat of the inside can with difficulty 

 pass through, while the luminous heat of the sun's rays on the con- 

 trary can penetrate with ease into the room. And thus we find the 

 use of these media which are at the same time diathermanons for 

 luminous rays, but athermanous for the rays of obscure heat. 



This double glazing may be very advantageously employed in 



conservatories, where the plants 

 require the full daylight, and 

 which pine when the glass is 

 covered by mats or other opaque 

 material to protect them from the 

 cold. 



Cellars are less exposed than 

 the rooms of the upper stories 

 to the exchange of heat which 

 takes place between the inside 

 and the outside or vice versd. 

 They thus preserve throughout 

 the year a medium temperature, 

 which varies the less as they are 

 situated deeper. So they appear 

 to be warm in winter and cool 

 in summer. Though we cannot 

 regard them as very healthy 

 places for living in, they serve 



at least for the preservation of many things which are damaged 

 either by excessive cold, or too great heat. 



Ice-houses are a sort of cellar rather deeper than ordinary ones, 

 dug in the ground, and cased inside by a wall of brick. Into 

 this pieces of ice are thrown during winter time that they may be 

 kept and used in the hot season. After the cavity is filled with frag- 

 ments of ice, a certain quantity of water is poured in during severe 

 frosts, which covers the whole with a layer of ice which prevents the 

 access of the outer air. Straw is then heaped up over it and foims a 



FIG. 244. An icehouse. 



