CLIMATOLOGY OF NEW HAMPSHIRE. 123 



the atmosphere can sustain : hence the relative humidity is 50 per cent. 

 Now, if the temperature rises just above 70, and the amount of moisture 

 is not increased, there is only one fourth the amount of moisture that the 

 atmosphere can sustain at that temperature ; hence the relative humidity 

 is 25 per cent. If, on the other hand, the temperature falls to 30, there 

 is more moisture than the atmosphere can sustain, and it is precipitated. 

 The air feels moist or dry, not from the absolute amount of moisture 

 present, but from its relative humidity. If the per-centage is small, the 

 moisture evaporates rapidly from the earth and from vegetation, as well 

 as from everything containing moisture. The opposite effect is seen on 

 the approach of rain. After a drouth, water is seen where there had been 

 none for weeks ; and the partially withered leaves assume their natural 

 shape, so much so that we should scarcely know that they had been 

 affected by a drouth : and all this before a drop of rain has fallen. Why ? 

 Because the air is approaching saturation, and moisture is no longer 

 evaporated from the earth and vegetation. 



The vapor of water diffused through the air is an obstruction to the 

 free passage of the heat of the sun, and also prevents the sudden radia- 

 tion of the heat that has been absorbed by the earth. "This," says 

 Buchan, "is undoubtedly one of the most important and conservative 

 functions of the invisible moisture of the atmosphere. For if the mois- 

 ture was drained out of it, and its diathermacy thereby rendered 

 complete, the sun's rays would burn up everything by their intolerable 

 fierceness ; " and during the night the escape of heat by radiation would 

 be so rapid that, ere the sun appeared again, everything would perish that 

 a freezing temperature could kill. We .see the effect of the want of 

 moisture frequently in New Hampshire, in the extremely hot days and 

 cold nights that invariably accompany a long drouth. 



EFFECT OF FORESTS. 



How far the removal or renewal of forests affects our climate, is some- 

 thing in which every one is interested. From the data that we have, 

 it may be impossible to generalize to any great extent ; yet there are some 

 things that we can learn from the observations that have been made. 

 While it is stoutly contended that there has been no decrease in the 

 annual amount of rain-fall in the eastern part of the United States, there 



