44 



AGRONOMY 



centigrade, subtract 32 and multiply by | (F. 32 x | = C.). 

 Below zero centigrade one multiplies by | and subtracts tin- 

 product from 32, if it is not more than 32. If more, the differ- 

 ence between this and 32 indicates the degrees below zero 

 Fahrenheit. If the temperatures below zero Fahrenheit are to 

 be changed to centigrade, add 32 and multiply by jj. The 

 result will be degrees below zero centigrade. In all cases 



FIG. 20. Mean annual rainfall in the United States 

 From Brigham's " Commercial Geography " 



where temperatures are recorded without the scale indicated, 

 as in this book, it is understood to refer to the Fahrenheit 

 scale. The centigrade scale, however, has found favor with 

 scientific men, and it is probable that in the near future it 

 will supplant the other. 



Precipitation. The most important function of the soil is to 

 afford water for plants. In many parts of the West thousands 

 of acres are barren and useless for want of this indispensable 

 factor. When it is applied to the soil in irrigation, the desert 

 at once becomes a garden. Precipitation may occur as rain, 



