20 THE WEATHER 



does not change, except possibly through long geological periods. 

 When the climate of a locality has been once determined, it may be 

 counted on absolutely. What the climate is for this generation it will 

 be for the next, and the next, so far as we can see. It could not be other- 

 wise, for climate in the large is the result of the sun's heat, modified 

 by the topography of the earth's surface the mountains, the valleys, 

 the oceans; and " so long as the sun shines with his accustomed 

 vigor and the hills and the seas abide in their places," so long will 

 the climate of every locality remain unchanged. The fact that crops 

 now are grown successfully in what are considered arid regions, and 

 are being pushed farther and farther into the frosty north, has been 

 cited in support of the contention that the climate is changing ; but 

 these changes in the area of successful production have not been 

 brought about by an increase of rainfall on the one hand, or of 

 temperature on the other, but by new methods of cultivation and 

 seed selection, and better adaptation of human practices to natural 

 conditions. 



We may rely, therefore, upon the permanency of the climatic factor 

 in crop production. The weather may vary by a small margin from 

 year to year, or from one season to the next, but the average tem- 

 perature, rainfall, and sunshine for so short a period as ten years will 

 depart so little from the true normal climate that the departure may 

 be neglected in actual practice. 



Climatic records compiled by the Weather Services. 



As it requires about ten years of careful observation to determine 

 approximately the average or normal temperature of a locality, and 

 perhaps twenty years to determine the normal rainfall, few farmers 

 would feel that they had the time or skill to devote to so serious an 

 undertaking ; nor is it necessary that they should. This work has been 

 done already in the United States, and with great accuracy and care. 

 The Weather Bureau of the United States Department of Agriculture 

 has collected and tabulated all records of temperature and rainfall 

 that have been made in the United States. Some of these records 

 cover a period of more than a hundred years, many of them more than 

 fifty years, and the work still is going on. At present, observations 

 are being made at about 4000 places. With this number of records, 

 distributed more or less evenly over the entire country, it is possible 



