162 EFFECTS OF HEAT. 



pulverize the soil, by admitting air into it, 

 and this increases its mean temperature. 



Q. Wliy are FRANCE and GERMANY WARMER 

 now, than when the vine would not ripen there ? 



A. Chiefly because their vast forests 

 have been cut down ; and the soil is better 

 drained and cultivated. 



Q,. What becomes of the WATER of PONDS and 

 TUBS in summer-time ? 



A. Ponds and tubs in summer-time 

 are often left dry, because their water is 

 evaporated by the air. 



Q,. How is this EVAPORATION PRODUCED and 

 carried on ? 



A. The air contains heat, and changes 

 the surface of the water into vapour; 

 this vapour (blending with the air) is 

 soon wafted away; whileyres^ portions of 

 air blow over the water, and produce a 

 similar evaporation; till the pond or tub 

 is left quite dry. 



Q,. Why are the WHEELS of some machines 



kept CONSTANTLY WET with WATER ? 



A. To carry off the heat (arising 

 from the rapid motion of the wheels) 

 by evaporation, as soon as it is developed. 



