'1HE lUmGA'HON AGE. 



terns executed by governments or corporations, such as are in opera- 

 tion in India, the United States, and in the valley of the Nile. Certain 

 of those systems are vast, and have been instituted under the pres- 

 sure of meeting great emergencies. To-day India is using irrigation 

 upon a stupendous scale in grappling with the calamity of famine. 



Economic irrigation requires the consideration of physical laws 

 which were unknown to the authors of primitive method-;, and waich 

 have not been generally observed in establishing the huge system of 

 irrigation already mentioned. Some of the physical laws which u nder- 

 lie any rational practice in the application of water to crops are briefly 

 considered in the following paper. 



EVAPORATION OF MOISTURE FROM WATEll SURFACES AND SOILS, 



The movement of moisture is constantly going on, The simplest 

 evidence of this movement is seen in rainfall and in the evaporation, 

 from water and soil surfaces. 



The factors that have been given the greatest prominence as exer- 

 cising a controlling action upon evaporation from soil and from the 

 surface of water are the temperature and the relative humidity of the 

 air- This view is amply sustained if the examination is confined to the 

 action of these factors during the extreme season of the year There 

 is no question concerning the greater evaporation of moisture from, 

 soils and waters during the months of summer, when temperatures are 

 high and the amountof atmospheric moisture is also relatively smaller 

 than during the cold season, when the temperature is lower and the 

 humidity of the air greater. This is demonstrated in many localities 

 by the excess of water that accumulates within and upon the soil in 

 winter and the droughts that, obtain in the summer. There are local- 

 ities and regions, however, that are so fortunate as to hwe the great- 

 est rainfall during the season of greatest evaporation and consequent- 

 ly the greatest plant growth. Setting aside the differences concur- 

 rent with the seasons and confining observations to the relative actions 

 of the several factors -during the summer, it is then found that the 

 temperature of the air and the amount of moisture that it contain ; 

 are not the most dominant factors in the control of evaporati m. As 

 already said, they are factors, but their combined effeats do not com- 

 pare with the effects of wind. Not only in the matter of irrigation, 

 but also in the location and exposure of reservoirs this fact is of lead- 

 ing importance. In view of this the writter carried out a series of 

 evaporation determinations by means of evaporators, at the same time- 

 keeping a record of the temperature and relative humidity of the air. 

 These observations were made as a part of a study of the factors that 

 control the rational irrigation of the sugar-cane on the Hawaiian 

 slands- The form of evaporator used was small agalvanized iron pan.. 



