142 GENERAL SCIENCE 



Conductivity of the Earth. Although the interior of 

 the earth is highly heated, the surface of the earth is very 

 little affected by it. As we go from the surface toward the 

 center of the earth we find that the ground is quite cold for 

 a few hundred feet. As we go still deeper the temperature 

 increases at the rate of about 1 C. for every 110 feet of 

 descent. 



Water pipes and drain pipes are placed a few feet under 

 ground so that they will be out of reach of the frost. 



Conductivity and Sensation. On a cold day metals 

 feel much colder than a piece of wood, although the tem- 

 perature of the wood be the same as the temperature of the 

 metal. On the other hand, if both bodies have been lying 

 in the hot sun the metal will seem much hotter to the hand 

 than the wood. The explanation is found in the fact 

 that the metal, being a much better conductor of heat than 

 wood, conveys the heat away from the hand much more 

 rapidly when it is cold, and conveys the heat to the hand 

 much more rapidly when it is hot, than does the wood. 

 If the metal is warmer than the body, it will feel hotter 

 than the wood ; if it is colder than the body, it will feel 

 colder than the wood. We speak of " warm blankets, " 

 when the blankets are evidently no warmer than the 

 surrounding objects. We simply mean that the blankets 

 are poor conductors of heat, and so they feel warmer than 

 the other objects. So also the rug feels much warmer than 

 a tiled floor or an oilcloth. 



The Fireless Cooker. This is merely an arrangement 

 of non-conducting substances to prevent the escape of 

 heat from heated foods placed in it (Figure 123). The two 

 walls of the fireless cooker are separated by a thick layer 

 of non-conducting materials such as sawdust, felt, or cork 

 shavings. These inclose a considerable amount of air, 



