CLIMATE OF HISTORIC TIMES 529 



thirteenth centuries after Christ. It is also found during the interval 

 from the culmination of a moist epoch to the culmination of a dry one, 

 for at such times conditions would be growing drier and the people would 

 be under stress. This was seemingly the case during the period from the 

 second to the fourth centuries of our era. North Africa and Syria may 

 have been moister than at present; but they were gradually becoming 

 drier, and the natural effect on a vigorous, competent people like the 

 Eomans was to cause them to construct all manner of engineering works 

 to provide the necessary water. 



The considerations which have just been set forth have led to a third 

 hypothesis, that of pulsatory climatic changes. According to this, the 

 earth's climate is not stable, nor does it change uniformly in one direc- 

 tion. It appears to fluctuate back and forth not only in the little waves 

 which we see from year to year or decade to decade, but in much larger 

 ones, which take hundreds of years or even a thousand. These in turn 

 seem to merge into and be imposed on the greater waves which form 

 Glacial stages. Glacial epochs, and Glacial periods. At the present time 

 there seems to be no way of determining whether the general -tendency is 

 toward aridity or toward glaciation. The seventh century of our era was 

 apparently the driest time during the historic period — distinctly drier 

 than the present — but the thirteenth century was almost equally dry, and 

 the twelfth or thirteenth before Christ may have been very dry. 



TREES AS A CLIMATIC YARDSTICK 



The best test of an hypothesis is actual measurements. In the case of 

 the pulsatory hypothesis we are fortunately able to apply this test. The 

 growth of vegetation depends on many factors — such as soil, exposure, 

 wind, sun, temperature, rain, and so forth. In the case of any individual 

 tree, however, the variations from year to year depend largely on climate. 

 The most critical factor for the great majority of plants is the amount of 

 moisture during the few months of most rapid growth. ^^ In the case of 

 trees, the work of Douglass^* and others has shown the thickness of the 

 annual rings affords a reliable indication of the amount of moisture avail- 

 able during the period of growth. This is especially true when the growth 

 of several years is taken as the unit and is compared with the growth of 



23 A most careful and convincing study of this problem is embodied in an article by 

 J. W. Smith : The effects of weather upon the yield of corn. Monthly Weather Review, 

 vol. 42, 1914, pp. 78-92. On the basis of the yield of corn in Ohio for 60 years and in 

 other States for shorter periods, he shows that the rainfall of July has almost as much 

 influence on the crop as has the rainfall of all other months combined. 



24 See chapter by A. E. Douglass in "The Climatic Factor" ; also article by M. N. 

 Stewart : "The Relation of Precipitation to Tree Growth," in The Monthly Weather 

 Review, vol, 41, 1913. 



