HEAT: DISTRIBUTION AND MEASUREMENT 77 



79. How the Air is Warmed. The air in contact with 

 a stove is warmed by conduction. Being lighter, it is 

 pushed away by cooler air and moves upward (convection) . 

 In a similar way, the atmosphere surrounding the earth is 

 warmed by heat from the earth. Besides distributing heat 

 by conduction and convection, the earth radiates heat. 

 This radiated heat passes through the air, if it is dry, without 

 raising its temperature; but if there is much vapor in the 

 air, the vapor prevents the passage of radiated heat and the 

 air retains the heat. A layer of cloud near the earth at 

 night is like a blanket keeping the earth warm. 



80. Intensity of Heat. The intensity of heat is stated in 

 terms of degrees of a thermometer. Water, oil, air, and iron 

 at a given degree (as at 10 C. or 50 F.) have the same 

 temperature or intensity of heat. 



Very high temperatures cannot be measured with an in- 

 strument made of glass because the glass would melt, but 

 they are sometimes determined by the amount of expansion 

 of a metal rod. For such measurements, a rod is fastened 

 at one end to a frame, and the other end is supported but 

 free to move. As the rod becomes heated, it lengthens and 

 pushes against a movable pointer. This pointer indicates, 

 on a scale, the increase in temperature. Such small differ- 

 ences as degrees are not indicated, but a change of a hundred 

 degrees or more is shown. Red-hot iron 'has a temperature 

 of 1,000 F. 



81. Quantity of Heat. A teacupful of water and a 

 pailful may have the same temperature, but the larger 

 amount of water would have the greater quantity of heat. 

 It would melt a greater weight of ice or raise the temperature 

 of a larger mass of matter a given number of degrees. 



82. The Calorie. The metric unit used in measuring the 

 quantity of heat in a body is called the calorie. It is the 

 quantity of heat required to warm one gram of water through 

 one degree Centigrade. For example, to warm one gram of 



