76 Generating Economic Cycles 



yield of the crops determined directly from the official 

 estimates of the annual yield of the crops since 1884. 

 The graphs are given in Figure. 20. 



Meteorological Cycles 



In 1919 the Carnegie Institution published Climatic 

 Cycles and Tree-Growth, by A. E. Douglass. With the 

 hope of throwing some light on the periodic activity of 

 the sun, Professor Douglass, by training and profession 

 an astronomer, undertook this laborious and valuable 

 investigation of the variation in the size of the rings of 

 trees, which, he assmned, was a consequence of me- 

 teorological changes having their origin in solar activity. 

 He selected for his first studies yellow pines taken from 

 the forests of northern Arizona. As there is, for the 

 most part, a scant and variable rainfall throughout the 

 state. Professor Douglass assumed as a working theory 

 (1) that the growth of pines in that rpgion depends 

 largely upon the available water; (2) that the rings of 

 the pines measure the growth of the trees; (3) that the 

 rings form a measure of annual precipitation. 



The rainfall of Arizona is extremely variable from 

 station to station. ''Storms come from the Pacific 

 coast and rain occurs a day or so later than in southern 

 California. Spring and autumn are the dry seasons, 

 and the warmest time of the year is usually in June, 

 just before the summer rains begin. The summer rains 

 occur in July and August and often come in 'spells ' 

 that last a week or two, with thunderstorms in the 

 afternoons or at night, followed by clear mornings. 

 Unlike the winter storms, the summer rains are local 



