CHAPTER II 

 CYCLES OF RAINFALL 



"The first thing that in my opinion ought to be done towards 

 making the observations useful for scientific purposes is to perform 

 that kind of more perfect averaging which is afforded by the har- 

 monic analysis. There is a certain amount of averaging done, but 

 that is chiefly daily averages, with monthly averages, and yearly 

 averages; but the more perfect averaging of the harmonic analysis 

 would give the level of the variation of the phenomenon." 



LORD KELVIN, in his testimony before the Meteorological Com- 

 mittee of the Royal Society, 1876. 



FROM the point of view of the relation of changing 

 weather to the varying fruit fulness of agriculture, the 

 most important factors that are usually included in 

 the term, weather, are temperature and rainfall. We 

 begin our investigation with this common belief and 

 inquire, in this chapter, whether the varying amount 

 of annual rainfall is subject to any simple law. 



In order to carry forward the inquiry as to the exist- 

 ence of a law of annual rainfall an analysis must be 

 made of a long record of precipitation. Our choice of 

 a record is limited by two conditions: First, our object 

 in investigating the periodicity of rainfall is the hope 

 of throwing light upon the periodicity in the yield of 

 the crops, and this expectation obviously makes it 

 desirable that the record of rainfall shall be as repre- 

 sentative as possible of the conditions of precipitation 



4 



