[12] C. G. Abbot and F. E. Fowle: Volcanoes and Climate; Smiths. Misc. Coll., Vol. 60, 1913, 24 pp.
W. J. Humphreys: Volcanic dust and other factors in the production of climatic and their possible relation to ice ages; Bull. Mount Weather Observatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air, 1920.
[13] H. Arctowski: The Pleonian Cycle of Climatic Fluctuations; Am. Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New York Academy of Sciences, Vol. 24, 1914.
[14] W. Köppen: Über mehrjährige Perioden der Witterung ins besondere üzer die II-jährige Periode der Temperatur. Also, Lufttemperaturen Sonnenflecke und Vulcanausbrüche; Meteorologische Zeitschrift, Vol. 7, 1914, pp. 305-328.
[15] The so-called sunspot numbers to which reference is made again and again in this book are based on a system devised by Wolf and revised by A. Wolfer. The number and size of the spots are both taken into account. The numbers from 1749 to 1900 may be found in the Monthly Weather Review for April, 1902, and from 1901 to 1918 in the same journal for 1920.
[16] Much of this chapter is taken from The Solar Hypothesis of Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.
[17] Ellsworth Huntington: Explorations in Turkestan, 1905; The Pulse of Asia, 1907; Palestine and Its Transformation, 1911; The Climatic Factor, 1915; World Power and Evolution, 1919.
[18] J. Hann: Klimatologie, Vol. 1, 1908, p. 352.
[19] H. C. Butler: Desert Syria, the Land of a Lost Civilization; Geographical Review, Feb., 1920, pp. 77-108.
[20] This is due to the fact that where these forests occur, in Gilead for example, the mountains to the west break down, so that the west winds with water from the Mediterranean are able to reach the inner range without having lost all their water. It is one of the misfortunes of Syria that its mountains generally rise so close to the sea that they shut off rainfall from the interior and cause the rain to fall on slopes too steep for easy cultivation.