WEATHER AND CLIMATE 147 



perature one degree, is so small that its change in volume when 

 confined in a small tube is apparent when the total quantity of heat 

 involved is relatively small. At the same time the coefficient of ex- 

 pansion is so small that a short tube only is needed for its increase 

 in volume from the freezing to the boiling temperature of water. 



From the coefficient of % 7B for the expansion (or contraction) of 

 gases, a thermometer scale has been devised having degrees of the same 

 value as those of the centigrade scale, but a zero 273 centigrade degrees 

 below the zero of the centigrade scale. Much use of this "absolute" 

 scale is made by scientists. 



Heat passes outward from any place where it is set free either by 

 conduction or convection in a material medium (solid, liquid, or gaseous 

 bodies), or by means of waves radiating outward through an ether 

 medium. 



Where bodies cool (lose heat) other than by conduction or convection, 

 it may be supposed their molecular energy or heat is used up in origi- 

 nating ether impulses or wave motions that pass off into surrounding 

 space. These in their turn may fall upon some body whose molecular 

 motion is quickened by the " absorption" of the ether motion, the 

 body thus becoming warmed. This transformation of ether motion 

 into molecular motion, or heat, is involved in the absorption by bodies 

 of light and other forms of radiant energy. 



Bodies may be kept hot, or "kept cold", by enclosing them in spaces 

 more or less free of air, or by filling confined air spaces with material 

 that neither conducts nor conveys heat readily. 



SOLAR HEATING 



The angular difference in direction of two lines from an 

 observer at any time, one directly upward (vertical) and one 

 toward the sun, may be considered as the obliquity of the 

 sun's rays at that particular time. This value is expressed 

 in degrees. If the sun at the time is halfway down from the 

 zenith (point directly overhead) to the horizon, the obliquity 

 of the sun's rays is 45. When the sun is at the horizon, the 

 angle is 90. Heat and light from the sun then may be con- 

 sidered as passing across the surface of the earth where the 

 observer is without being stopped by the earth's surface. 



