RELATIONS OF TREES TO TEMPERATURE. 205 



suits of all that science has yet discovered in relation to 

 the temperature of woods. But the effects of clearing the 

 forest are so different in different situations as to have 

 given origin to a multitude of theories. This diversity 

 of opinion, however, comes from a partial observation of 

 facts, without their qualifying circumstances. On a hot 

 summer's day we sprinkle our floors with water, for the 

 purpose of cooling the air of the room. But how can it 

 produce this effect, when by evaporation it carries heat from 

 the floor into the very air that is cooled by it ? The fact 

 is easily explained. The greater coolness felt when the 

 air of the room is saturated with the moisture evaporated 

 from the sprinkled floor might not be exactly indicated by 

 the thermometer. The sensation of coolness is caused by 

 the increased power of the air to conduct the heat rap- 

 idly from our persons, the effect of its greater humid- 

 ity. By the same law we may explain why, after a few 

 clear cold days in the winter, if a south-wind arises, we 

 feel as if the cold were greater, because this wind, while 

 it raises the temperature, charges the air with invisible 

 moisture. 



The coldness of the atmosphere over grassy meadows 

 when the sky is clear, after the decline of the sun in 

 summer, is a matter of common observation. As this 

 phenomenon is most evident on the clearest nights, it has 

 given rise to the notion that the moon cools the night air. 

 In our rambles after sunset, we have all felt these con- 

 stant changes of temperature, which are remarkable when 

 walking over an uneven road, the degree of heat cor- 

 responding nearly with our altitude. When we occupy 

 high ground, the air is warm and dry ; as soon as we de- 

 scend into a valley, we feel a sudden chill. These differ- 

 ences are not observed on a cloudy night, or when a clear 

 brisk wind is blowing. But in a calm state of the at- 

 mosphere, as the lowest stratum of air contains the great- 



