[21] H. Leiter: Die Frage der Klimaanderung waherend geschichtlicher Zeit in Nordafrika. Abhandl. K. K. Geographischen Gesellschaft, Wien, 1909, p. 143.
[22] 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. See his Agricultural Meteorology, New York, 1920.
[23] See chapter by A. E. Douglass in The Climatic Factor; and his book on Climatic Cycles and Tree-Growth; Carnegie Inst., 1919. Also article by M. N. Stewart: The Relation of Precipitation to Tree Growth, in the Monthly Weather Review, Vol. 41, 1913.
[24] The dotted line is taken from Palestine and Its Transformation, pp. 327 and 403.
[25] M. A. Stein: Ruins of Desert Cathay, London, 1912.
[26] In the preparation and interpretation of this table the help of Mr. G. B. Cressey is gratefully acknowledged.
[27] For the tree data used in these comparisons, see The Climatic Factor P. 328, and A. E. Douglass: Climatic Cycles and Tree Growth, p. 123.
[28] One year interpolated.
[29] J. W. Gregory: Is the Earth Drying Up? Geog. Jour., Vol. 43, 1914, pp. 148-172 and 293-318.
[30] Geog. Jour., Vol. 43, pp. 159-161.