40 BIOLOGICAL LECTURES. 



valley begin to fall shortly before sunset, but continue with 

 such unequal rapidity that the difference is very great imme- 

 diately before sunrise. Thus, for instance, on the morning of 

 Sept. 1 8, 1898, the temperature of the hilltop was 62° F. at 

 6.45, while that of the valley was 35° F., showing a differ- 

 ence of 27° F. between two points which differed only 100 

 meters in elevation, and which were actually so near each 

 other that one could walk from one instrument to the other in 

 ten minutes. 



If the curves of the thermograph are examined, it will be 

 found that the valley becomes from 3 to 8° warmer than the 

 hilltops during the middle of the day. 



These differences of temperature of the valleys and highlands 

 are due to the action of currents of air set up by the inversions 

 of temperature and by the direct action of the sun's rays. 



If the surface of a region were perfectly level, the surface 

 layer of air cooled by inversion would remain in place, and a 

 fairly uniform temperature would prevail over the entire area. 

 The cooling of the lower layer of air results in its contraction 

 and consequent increase in weight, with the result that in 

 regions with broken or irregular topography the cold air on the 

 mesas, ridges, and hilltops flows down the slopes into the 

 caiions, valleys, or other depressions, forming a deep layer, 

 while a constant supply of warmer air settles down on the high- 

 lands, with the result that these minor elevations have a higher 

 average temperature and a more equable climate than the 

 valleys below. 



The influence of this drainage of cold air has long been 

 recognized by the farmer and horticulturist in the northern and 

 New England states. Thus it is customary to select ridges and 

 uplands for the growth of vineyards, orchards, and small fruit 

 plantations in broken countries, since these places are less 

 subject to frost than the lowlands. 



If the air cooled by inversion on the highlands descends by 

 long, continuous, steep slopes and drains into narrow valleys, 

 it may exercise exactly the reverse influence upon the tempera- 

 ture of the valley. As it descends it will increase in tem- 

 perature at the normal adiabatic rate, by compression, and 



