286 STUDIES IN THE FIELD AND FOREST. 



autumnal period, if it comes at all, is a brief period of 

 warm weather, that sometimes greets our climate in 

 November, after the fall of the leaf, and not as many- 

 suppose in October. It is probably caused by the sud- 

 den check given to vegetable perspiration, by the fall of 

 the leaves. It is well known that by sprinkling a floor, 

 to cool a room in hot weather, we cause the heat to be 

 carried off with the evaporation of the water. On the 

 same principle, the infinite host of trees, whose leaves 

 are constantly evaporating the moisture of the earth, 

 must proportionally cool its surface, and the atmosphere 

 that is in contact with it. Any thing that increases 

 evaporation from the earth's surface must cool it in the 

 same manner. Hence we may explain the greater cold- 

 ness of the air over valleys and wet places on summer 

 evenings, and the fact, often noticed, that a rainy spell 

 in autumn is commonly succeeded by severe frosts. 

 The greater burden of the foliage of our woods remains 

 on the trees and shrubs, until the severe frosts in the 

 latter part of October. About this time the whole 

 extent of our forests is often laid bare in the brief space 

 of a week or ten days. Not only does this great extent 

 of surface, thus laid open to the sun, receive from his 

 rays an increased amount of heat, but there is a vast 

 and sudden diminution, at the same time, of that evap- 

 oration which is caused by the leaves of plants. These 

 two circumstances unite in producing, when no out- 

 ward agencies interfere, a great accumulation of heat. 

 The warm spell that follows is the true Indian summer, 

 and may last from five to eight days. During one of 

 these spells of fine weather, I have sometimes heard the 

 crickets chirping merrily as late as the eighteenth of 

 November. 



But our climate is exposed to such a variety of influ- 



