WEATHER AND CLIMATE 145 



or heat. The warmed body in turn may lose this energy of 

 its molecules by conduction to other bodies, by convection 

 currents, and by radiations from itself as an original source. 



Some substances are much better conductors of heat than 

 others. Some conduct heat so slowly as to be called non- 

 conductors. In fireless cookers the space between the 

 double walls is filled in with asbestos, felt, or some like non- 

 conducting material, thus preventing the loss outward of the 

 heat (molecular motion) of the material put into the inner 

 receptacle. This permits the softening process, or "cook- 

 ing' 7 of any foodstuff to continue for a long time after the 

 heated material has been properly packed away in the 

 cooker. The fireless cooker may likewise be used to keep 

 things cool in summer by keeping out the heat of the surround- 

 ing air. Icehouses are constructed on the same principle, 

 the space between their double walls being packed with some 

 cheap non-conducting material such as saw-dust, thus pre- 

 venting heat from without reaching and melting the ice. 



Refrigerators for household use, fire-proof safes, and 

 vaults for storage of business papers and other valuables 

 are built with double walls filled with non-conducting 

 material of some kind. In " thermos bottles" air was 

 withdrawn from between the double walls of glass, and that 

 space was then sealed air-tight by melting off the tubular 

 outlet of glass. Transmission of heat either inward or out- 

 ward through this vacuum is thus made impossible by either 

 conduction or convection. The jacket of hot-air furnaces, 

 and the pipes carrying heated air from the furnace to the 

 rooms to be warmed, are covered commonly with thin 

 sheets of asbestos to lessen wastage of heat in its trans- 

 mission. Steam and hot-water pipes are protected "from 

 the cold" in some like manner. 



It is to be remembered that the rise of heated air through 

 the smoke pipe and chimney, constituting the "draft" of a 



