158 



GENERAL SCIENCE 



it receives by conduction from the earth. The energy 

 required to change water to vapor appears again in the 

 form of heat when the vapor changes back to water and 

 falls as rain. This heat is considerable, 536 calories for 

 each gram of water. With snow the effect is still more 

 noticeable. Why? 



Not all the heat which reaches the earth is retained. 

 A large part of that which is reflected and radiated by the 

 earth passes back through the atmosphere into the endless 

 space beyond. 



Thermograph. Figure 133 shows a temperature curve 

 or a record of the variation in temperature of a given 



Flat bulb filled 

 with alcohol 



FIG. 134. Thermograph. 



This instrument is so constructed that the temperature is recorded 

 automatically. 



place for a given length of time. Such a curve may be 

 produced automatically with great accuracy by a thermo- 

 graph (Figure 134). 



Experiment 47. Take four or more thermometer readings at 

 the same time each day for one week and plot the temperature 

 curve. 



Isotherms. Lines which are drawn on the weather 

 maps connecting points having the same temperature 

 are called isotherms. There is considerable variation in 



