Effect of Forests upon Rainfall and Temperature. 21 



9. Lysi meter. 



Relation between the Rainfall and the Native Forests. 



85. As a general rule, we find our native forests more dense in 

 proportion as the rainfall is greatest, as 



we see proved upon the Pacific Coast, 

 and in the region south of Lake Supe- 

 rior. They become less as the rains 

 diminish, and as we approach the great 

 plains, the native timber is found only 

 along the borders of the rivers and 

 smaller streams, and finally it disap- 

 pears altogether. Asa rule, where the 

 amount of rain is less than twenty 

 inches in a year, and this chiefly in 

 winter, the growth of trees becomes 

 difficult, and with many species impos- 

 sible. 



86. There is some reason to believe 



that the capacity for cultivation in a dry region may be increased 

 by tree-planting, and gradually extended to a degree that would not 

 be possible to secure at first. 



Effect of Woodlands upon the Temperature of the Air and the 



Earth. 



87. It is evident to the senses that the air in woodlands 

 is cooler than in the open fields in summer and warmer 

 in winter, although the actual difference in the latter is 

 slight. When we measure the temperature of the soil, we 

 find the effect of woodlands much greater, the difference 

 between winter and summer being less in the woods than 

 in the fields, and less at greater depths than at the surface. 

 This difference is greatest in summer, when vegetation is 

 most active. Various means are employed to measure 

 this temperature. In one, a thermometer with a thick 

 glass bulb (invented by Lamont, of Munich), is left to 

 various depths, and when drawn up for observation it does 

 not quickly change before reading. In other cases, long- 

 stemmed instruments are permanently buried at different 

 depths, with the scales above the surface. 



