206 EFFECTS OF FORESTS ON SPRINGS AND RIVERS. 



former case the average temperature is |^° below that of unwooded 

 lands at a distance of 5 feet above the surface of the ground, 



" 3. That the average temperature of the spring within the forest 

 was 2-95*^ below that of unwooded land. This difference was less 

 apparent among shade-trees, than where pines were the growth. 



" 4. That the average temperature of summer in the day-time, in 

 thick woods, is 3*78° below that of unwooded land. 



" 5. That the temperature of the atmosphere within the woods, in 

 the summer season, increased 3 '94° from the ground to the tops of 

 the trees. 



" 6. That the average temperature of autumn within the woods 

 was scarcely 1^ below that of the open land ; and 



" 7. That in winter this difference in temperature almost entirely 

 disappeared. 



" These briefly mentioned differences between the temperatui-e of 

 the atmosphere within the forests and that of unwooded lands, affect 

 the constant current of air in the day-time. For instance, the stream 

 of air flows from the surface of the ground withiu the forests toward 

 the periphery of the woods, and thence spreads over the open fields. 

 It afterward passes back again to the crowns of the trees, and by 

 coniing in contact with the leaves which, during the day, are colder 

 than the atmosphere, the latter itself grows cooler and heavier, and so 

 gradually descends from the tops of the trees to the surface of the 

 ground. 



" At night the conditions are entirely different. The thermometer 

 now is higher in the woods than in the open lands. The Bavarian 

 observations for the month of July show that, while at midday the 

 temperature within the forest is 8'01° below that of the unwooded 

 land, at night it is 4*39 higher, (18 and 29 per cent.); in August, 

 during the afternoon, 7* 13° lower, and at night 3*71 higher, (16 and 

 22 per cent.) During the night, therefore, the colder and denser air 

 of the unwooded land passes into the forest, displaces the warmer air, 

 which rises and is cooled by contact with the leaves, and then 

 radiates. 



" The maximum and minimum temperature of the open lands is 

 never reached in the forest. The [mean ?] atmospheric temperature 

 of the woods always remains several degrees lower. 



" From these data, therefore, it is evident that the thermal effects 

 of woodland are of great moment, and that climatic alterations must 

 result from deforestation on a large scale ; that these climatic changes 

 will cousisi, as a general thing, in increasing the annual temperature, 



