SOIL MOISTURE AND DRY FARMING 95 



in the air, and this in turn should tend to lessen the in- 

 jury done by the hot winds that sometimes prevail in 

 dry areas. This increase in transpiration has led to the 

 hope that it will result in an increase in the precipitation, 

 but the evidence based on the results does not sustain 

 this view. The influence emanating from this increased 

 transpiration does not appear to be enough to affect the 

 precipitation, at least to any very appreciable extent. 



Other influences that affect evaporation. Among 

 the influences that affect evaporation in addition to those 

 that have been dwelt upon are: (1) the influence result- 

 ing from latitude; (2) the influence resulting from alti- 

 tude, and (3) that resulting from the store of humus in 

 the soil. In the discussion of this question these influ- 

 ences cannot be ignored, because of the important bear- 

 ing which they exercise upon evaporation. 



Latitude influences evaporation because of the in- 

 fluence which it exerts upon temperature. Evaporation 

 increases with increase in the temperature. This ex- 

 plains why evaporation is greatest when the summer 

 heat is greatest, other things being equal, and why it is 

 least in cool and cold weather. The loss of soil 

 moisture, therefore, in northern latitudes, will be pro- 

 portionately increased, other things being equal, with 

 increase in the temperature which follows as the result 

 of the lower latitude of the locality. 



The influence of altitude is probably no less potent 

 than that of latitude. With increase in the altitude 

 comes decrease in the temperature, and with decrease 

 in the temperature comes a lessened transpiration. Ele- 

 vation alone may result in protecting a crop from the 

 baneful influences of a temperature that will wither the 

 same in lower altitudes, notwithstanding that these may 

 be in the same latitude. Thus it is that betimes a crop 

 will be withered in a low valley by hot winds which do 

 not harm the same on a high altitude in proximity there- 



