206 EELATIONS OF TREES TO TEMPEEATUEE. 



est amount of moisture, its capacity for retaining heat is 

 proportionally diminished. Consequently the heat from 

 the ground is radiated with great rapidity through this 

 damp stratum of air, while the higher strata remain un- 

 changed in their temperature. Indeed, it has been found 

 by experiment that while the greatest heat at noonday in 

 calm summer weather is very near the surface of the 

 ground, yet after dew-fall the highest temperature is 

 several feet above this surface, iacreasing in altitude for 

 some hours after sunset. 



The action of a wood checks .this radiation in the early 

 part of the night. Like clouds in the evening, the trees 

 form a canopy of foliage over the ground, and thereby 

 retain the heat many hours after it has escaped by ra- 

 diation in the open plain. According to these laws of 

 the radiation of heat, a longer time would be rec[uired to 

 cool a tract of forest than an equal area of open space, 

 down to a given point. But, on the other hand, a pro- 

 portionally longer time is required to raise the tempera- 

 ture in the woods to a given point. Hence it is still a 

 question among meteorologists whether the mean annual 

 temperature of a large tract of country is higher or lower 

 when covered with forest than when generally open and 

 cleared. The sun acts with greater force upon an open 

 country ; but the radiation of heat is greater in the same 

 ratio during the sun's absence. 



In considering the effects of clearing, travellers have 

 often overlooked the important advantages of protection 

 afforded by woods to agricultural crops. Even if the mean 

 annual temperature of a country be the same after it is 

 cleared as when it was covered, it may at the same time 

 be too cold for certain plants which were formerly its 

 common productions, because there are no woods to pro- 

 tect them from the winds by day or from the cold caused 

 by excessive radiation at night. Palestine, two thousand 



