[1] W. A. Setchell: The Temperature Interval in the Geographical Distribution of Marine AlgÆ; Science, Vol. 52, 1920, p. 187.

[2] J. Barrell: Rhythms and the Measurements of Geologic Time; Bull. Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.

[3] Pirsson and Schuchert: Textbook of Geology, 1915, pp. 538-550.

[4] From Charles Schuchert in The Evolution of the Earth and Its Inhabitants: Edited by R. S. Lull, New Haven, 1918, but with revisions by Professor Schuchert.

[5] J. H. Poynting: Radiation in the Solar System; Phil. Trans. A, 1903, 202, p. 525.

[6] L. J. Henderson: The Fitness of the Environment, 1913.

[7] Henderson: loc. cit., p. 138.

[8] F. R. Moulton: Introduction to Astronomy, 1916.

[9] Moulton: loc. cit.

[10] James Croll: Climate and Time, 1876.

[11] T. C. Chamberlin: An attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric basis; Jour. Geol., Vol. VII, 1899, pp. 545-584, 667-685, 757-787.

T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp. 93-106, 655-677, and Vol. III, pp. 432-446.

S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some investigations on carbon dioxide which have had a pronounced effect on later conclusions.

F. Frech adopted Arrhenius' idea and developed it in a paper entitled Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte Rendu, Tenth (Mexico) Congr. Geol. Intern., 1907 (=1908), pp. 299-325.

The exact origin of the carbon dioxide theory has been stated so variously that it seems worth while to give the exact facts. Prompted by the suggestion, of Tyndall that glaciation might be due to depletion of atmospheric carbon dioxide, Chamberlin worked up the essentials of his early views before he saw any publication from Arrhenius, to whom the idea has often been attributed. In 1895 or earlier Chamberlin began to give the carbon dioxide hypothesis to his students and to discuss it before local scientific bodies. In 1897 he prepared a paper on "A Group of Hypotheses Bearing on Climatic Changes," Jour. Geol., Vol. V (1897), to be read at the meeting of the British Association at Toronto, basing his conclusions on Tyndall's determination of the competency of carbon dioxide as an absorber of heat radiated from the earth. He had essentially completed this when a paper by Arrhenius, "On the influence of carbonic acid in the air upon the temperature of the ground," Phil. Mag., 1896, pp. 237-276, first came to his attention. Chamberlin then changed his conservative, tentative statement of the functions of carbon dioxide to a more sweeping one based on Arrhenius' very definite quantitative deductions from Langley's experiments. Both Langley and Arrhenius were then in the ascendancy of their reputations and seemingly higher authorities could scarcely have been chosen, nor a finer combination than experiment and physico-mathematical development. Arrhenius' deductions were later proved to have been overstrained, while Langley's interpretation and even his observations were challenged. Chamberlin's latest views are more like his earlier and more conservative statement.

[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.

[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.

[31] See A. J. Henry: Secular Variation of Precipitation in the United States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.

[32] O. Pettersson: The connection between hydrographical and meteorological phenomena; Quarterly Journal of the Royal Meteorological Society, Vol. 38, pp. 174-175.

[33] A. Norlind: Einige Bemerkungen Über das Klima der historischen Zeit nebst einem Verzeichnis mittelaltlicher Witterungs erscheinungen; Lunds Univ. Arsskrift, N. F., Vol. 10, 1914, 53 pp.

[34] Thorwald Rogers: A History of Agriculture and Prices in England.

[35] E. BrÜckner: Klimaschwankungen seit 1700, Vienna, 1891.

[36] For a full discussion of the changes in the Caspian Sea see The Pulse of Asia, pp. 329-358.

[37] S. Q. Morley: The Inscriptions at CopÁn; Carnegie Inst. of Wash., No. 219, 1920.

Ellsworth Huntington: The Red Man's Continent, 1919.

[38] See summary of Wolf's work with additional information by H. Fritz; ZÜrich Vierteljahrschrift, Vol. 38, 1893, pp. 77-107.

[39] This chapter is an amplification and revision of the sketch of the glacial period contained in The Solar Hypothesis of Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.

[40] R. D. Salisbury: Physical Geography of the Pleistocene, in Outlines of Geologic History, by Willis, Salisbury, and others, 1910, p. 265.

[41] The Quaternary Ice Age, 1914, p. 364.

[42] For fuller discussion of climatic controls see S. S. Visher: Seventy Laws of Climate, Annals Assoc. Am. Geographers, 1922.

[43] Many of these alterations are implied or discussed in the following papers:

1. F. W. Harmer: Influence of Winds upon the Climate of the Pleistocene; Quart. Jour. Geol. Soc., Vol. 57, 1901, p. 405.
2. C. E. P. Brooks: Meteorological Conditions of an Ice Sheet; Quart. Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70, and The Evolution of Climate in Northwest Europe; op. cit., Vol. 47, 1921, pp. 173-194.
3. W. H. Hobbs: The RÔle of the Glacial Anticyclone in the Air Circulation of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 185-225.

[44] W. B. Wright: The Quaternary Ice Age, 1914, p. 100.

[45] The description of the distribution of the ice sheet is based on T. C. Chamberlin's wall map of North America at the maximum of glaciation, 1913.

[46] Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H. Hobbs: Characteristics of Existing Glaciers, 1911.

[47] S. S. Visher: The Geography of South Dakota; S. D. Geol. Surv., 1918.

[48] W. H. Hobbs: Characteristics of Existing Glaciers, 1911. The RÔle of the Glacial Anticyclones in the Air Circulation of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 185-225.

[49] R. D. Salisbury: Physiography, 1919.

[50] Griffith Taylor: Australian Meteorology, 1920, p. 283.

[51] J. D. Whitney: Climatic Changes of the Later Geological Times, 1882.

[52] E. E. Free: U. S. Dept. of Agriculture, Bull. 54, 1914. Mr. Free has prepared a summary of this Bulletin which appears in The Solar Hypothesis, Bull. Geol. Sec. of Am., Vol. 25, pp. 559-562.

[53] G. K. Gilbert: Lake Bonneville; Monograph 1, U. S. Geol. Surv.

[54] C. E. P. Brooks: Quart. Jour. Royal Meteorol. Soc., 1914, pp. 63-66.

[55] H. J. L. Beadnell: A. Egyptian Oasis, London, 1909.

Ellsworth Huntington: The Libyan Oasis of Kharga; Bull. Am. Geog. Soc., Vol. 42, Sept., 1910, pp. 641-661.

[56] S. S. Visher: The Bajada of the Tucson Bolson of Southern Arizona; Science, N. S., Mar. 23, 1913.

Ellsworth Huntington: The Basins of Eastern Persia and Seistan, in Explorations in Turkestan.

[57] Griffith Taylor: Australian Meteorology, 1920, p. 189.

[58] Chamberlin and Salisbury: Geology, 1906, Vol. III, pp. 405-412.

[59] It may have retreated soon after reaching its maximum. If so, the general lack of thick terminal moraines would be explained. See page 122.

[60] Rollin T. Chamberlin: Personal Communication.

[61] F. H. Knowlton: Evolution of Geologic Climates; Bull. Geol. Soc. Am., Vol. 30, 1919, pp. 499-566.

[62] Chas. Schuchert: Review of Knowlton's Evolution of Geological Climates, in Am. Jour. Sci., 1921.

[63] G. R. Wieland: Distribution and Relationships of the Cycadeoids; Am. Jour. Bot., Vol. 7, 1920, pp. 125-145.

[64] D. T. MacDougal: Botanical Features of North American Deserts; Carnegie Instit. of Wash., No. 99, 1908.

[65] Loc. cit.

[66] H. H. Clayton: Variation in Solar Radiation and the Weather; Smiths. Misc. Coll., Vol. 71, No. 3, Washington, 1920.

[67] B. Helland Hansen and F. Nansen: Temperature Variations in the North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths. Inst., Vol. 70, No. 4, Washington, 1920.

[68] The climatic significance of ocean currents is well discussed in Croll's Climate and Time, 1875, and his Climate and Cosmogony, 1889.

[69] F. J. B. Cordeiro: The Gyroscope, 1913.

[70] W. W. Garner and H. A. Allard: Flowering and Fruition of Plants as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp. 377-400.

[71] Report of Committee on Sedimentation, National Research Council, April, 1922.

[72] Chas. Schuchert: The Earth's Changing Surface and Climate during Geologic Time; in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 55.

[73] Quoted by J. Cornet: Cours de GÉologie, 1920, p. 330.

[74] T. C. Chamberlin: The Order of Magnitude of the Shrinkage of the Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.

[75] G. I. Taylor: Philosophical Transactions, A. 220, 1919, pp. 1-33; Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, p. 308.

[76] J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80, p. 309.

[77] E. W. Brown: personal communication.

[78] C. S. Slichter: The Rotational Period of a Heterogeneous Spheroid; in Contributions to the Fundamental Problems of Geology, by T. C. Chamberlin, et al., Carnegie Inst. of Wash., No. 107, 1909.

[79] E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.

[80] Chas. Schuchert: The Earth's Changing Surface and Climate; in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 78.

[81] J. Barren: Rhythms and the Measurement of Geologic Time; Bull. Geol. Soc. Am., Vol. 28, 1917, p. 838.

[82] Chas. Schuchert: loc. cit., p. 78.

[83] T. C. Chamberlin: Diastrophism, the Ultimate Basis of Correlation; Jour. Geol., Vol. 16, 1909; Chas. Schuchert: loc. cit.

[84] Pirsson-Schuchert: Textbook of Geology, 1915, Vol. II, p. 982; Chas. Schuchert: Paleogeography of North America; Bull. Geol. Soc. Am., Vol. 20, pp. 427-606; reference on p. 499.

[85] The general subject of the climatic significance of continentality is discussed by C. E. P. Brooks: continentality and Temperature; Quart. Jour. Royal Meteorol. Soc., April, 1917, and Oct., 1918.

[86] Chas. Schuchert: Climates of Geologic Time; in The Climatic Factor; Carnegie Institution, 1914, p. 286.

[87] A. de Lapparent: TraitÉ de GÉologie, 1906.

[88] Chas. Schuchert: Historical Geology, 1915, p. 464.

[89] M. M. Metcalf: Upon an important method of studying problems of relationship and of geographical distribution; Proceedings National Academy of Sciences, Vol. 6, July, 1920, pp. 432-433.

[90] Chas. Schuchert: Paleogeography of North America; Bull. Geol. Soc. Am., Vol. 20, 1910; and Willis, Salisbury, and others: Outlines of Geologic History, 1910.

[91] Chas. Schuchert: The Earth's Changing Surface and Climate; in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 50.

[92] A. J. Henry: The Decrease of Precipitation with Altitude; Monthly Weather Review, Vol. 47, 1919, pp. 33-41.

[93] Chas. F. Brooks: Monthly Weather Review, Vol. 46, 1918, p. 511; and also A. J. Henry and others: Weather Forecasting in the United States, 1913.

[94] F. H. Knowlton: Evolution of Geologic Climates; Bull. Geol. Soc. Am., Vol. 30, Dec., 1919, pp. 499-566.

[95] Talbert, quoted by I. Bowman: Forest Physiography, 1911, p. 63.

[96] J. Barrell: Rhythms and the Measurement of Geologic Time; Bull. Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.

[97] C. E. P. Brooks: The Evolution of Climate in Northwest Europe. Quart. Jour. Royal Meteorol. Soc., Vol. 47, 1921, pp. 173-194.

[98] H. F. Osborn: Men of the Old Stone Age, N. Y., 1915; J. M. Tyler: The New Stone Age in Northwestern Europe, N. Y., 1920.

[99] EncyclopÆdia Britannica, 11th edition: article "Ocean."

[100] C. E. P. Brooks: The Meteorological Conditions of an Ice sheet and Their Bearing on the Desiccation of the Globe; Quart. Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70.

[101] Data of Geochemistry, Fourth Ed., 1920; Bull. No. 695, U. S. Geol. Survey.

[102] Quoted by Schuchert in The Evolution of the Earth.

[103] Smithsonian Physical Tables, Sixth Revision, 1914, p. 142.

[104] Chamberlin, in a very suggestive article "On a possible reversal of oceanic circulation" (Jour. of Geol., Vol. 14, pp. 363-373, 1906), discusses the probable climatic consequences of a reversal in the direction of deep-sea circulation. It is not wholly beyond the bounds of possibility that, in the course of ages the increasing drainage of salt from the lands not only by nature but by man's activities in agriculture and drainage, may ultimately cause such a reversal by increasing the ocean's salinity until the more saline tropical portion is heavier than the cooler but fresher subpolar waters. If that should happen, Greenland, Antarctica, and the northern shores of America and Asia would be warmed by the tropical heat which had been transferred poleward beneath the surface of the ocean, without loss en route. Subpolar regions, under such a condition of reversed deep-sea circulation, might have a mild climate. Indeed, they might be among the world's most favorable regions climatically.

[105] EncyclopÆdia Britannica: article "Ocean."

[106] Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132, 1906; and T. C. Chamberlin: The Origin of the Earth, 1916.

[107] Personal communication.

[108] R. T. Chamberlin: Gases in Rocks, Carnegie Inst. of Wash., No. 106, 1908.

[109] J. Barrell: The Origin of the Earth, in Evolution of the Earth and Its Inhabitants, 1918, p. 44, and more fully in an unpublished manuscript.

[110] F. W. Clarke: Data of Geochemistry, Fourth Ed., 1920, Bull. No. 695, U. S. Geol. Survey, p. 256.

[111] F. W. Clarke: loc. cit., pp. 27-34 et al.

[112] Chas. E. St. John: Science Service Press Reports from the Mt. Wilson Observatory, May, 1922.

[113] Abbot and Fowle: Annals Astrophysical Observatory; Smiths. Inst., Vol. II, 1908, p. 163.

F. E. Fowle: Atmospheric Scattering of Light; Misc. Coll. Smiths. Inst., Vol. 69, 1918.

[114] Abbot and Fowle: loc. cit., p. 172.

[115] H. H. Turner: On a Long Period in Chinese Earthquake Records; Mon. Not. Royal Astron. Soc., Vol. 79, 1919, pp. 531-539; Vol. 80, 1920, pp. 617-619; Long Period Terms in the Growth of Trees; idem, pp. 793-808.

[116] Harlow Shapley: Note on a Possible Factor in Geologic Climates; Jour. Geol., Vol. 29, No. 4, May, 1921; NovÆ and Variable Stars, Pub. Astron. Soc. Pac., No. 194, Aug., 1921.

[117] J. H. Jeans: Problems of Cosmogony and Stellar Dynamics, Cambridge, 1919.

[118] This fact is so important and at the same time so surprising to the layman, that a quotation from The Electron Theory of Matter by O. W. Richardson, 1914, pp. 326 and 334 is here added.

"It is a very familiar fact that when material bodies are heated they emit electromagnetic radiations, in the form of thermal, luminous, and actinic rays, in appreciable quantities. Such an effect is a natural consequence of the electron and kinetic theories of matter. On the kinetic theory, temperature is a measure of the violence of the motion of the ultimate particles; and we have seen that on the electron theory, electromagnetic radiation is a consequence of their acceleration. The calculation of this emission from the standpoint of the electron theory alone is a very complex problem which takes us deeply into the structure of matter and which has probably not yet been satisfactorily resolved. Fortunately, we can find out a great deal about these phenomena by the application of general principles like the conservation of energy and the second law of thermodynamics without considering special assumptions about the ultimate constitution of matter. It is to be borne in mind that the emission under consideration occurs at all temperatures although it is more marked the higher the temperature.... The energy per unit volume, in vacuo, of the radiation in equilibrium in an enclosure at the absolute temperature, T, is equal to a universal constant, A, multiplied by the fourth power of the absolute temperature. Since the intensity of the radiation is equal to the energy per unit volume multiplied by the velocity of light, it follows that the former must also be proportional to the fourth power of the absolute temperature. Moreover, if E is the total emission from unit area of a perfectly black body, we see from p. 330 that E=A´T⁴, where A´ is a new universal constant. This result is usually known as Stefan's Law. It was suggested by Stefan in the inaccurate form that the total radiant energy of emission from bodies varies as the fourth power of the absolute temperature, as a generalization from the results of experiments. The credit for showing that it is a consequence of the existence of radiation pressure combined with the principles of thermodynamics is due to Bartoli and Boltzmann."

[119] Quoted by Moulton in his Introduction to Astronomy.

[120] Introduction to Astronomy.

[121] The term billions, here and elsewhere, is used in the American sense, 10⁹.

[122] The assumed number of stars here is ten times as great as in the other parts of this line.

[123] Lewis Boss: Convergent of a Moving Cluster in Taurus; Astronom. Jour., Vol. 26, No. 4, 1908, pp. 31-36.

[124] F. R. Moulton: in Introduction to Astronomy, 1916.

[125] A. Penck: Die Alpen im Eiszeitalter, Leipzig, 1909.

[126] R. D. Salisbury: Physical Geography of the Pleistocene, in Outlines of Geologic History, by Willis and Salisbury, 1910, pp. 273-274.

[127] Davis, Pumpelly, and Huntington: Explorations in Turkestan, Carnegie Inst. of Wash., No. 26, 1905.

In North America the stages have been the subject of intensive studies on the part of Taylor, Leverett, Goldthwait, and many others.

[128] Double star.

[129] E. Kirk: Paleozoic Glaciation in Alaska; Am. Jour. Sci., 1918, p. 511.

[130] J. Milne: Catalogue of Destructive Earthquakes; Rep. Brit. Asso. Adv. Sci., 1911.

[131] Wm. Bowie: Lecture before the Geological Club of Yale University. See Am. Jour. Sci., 1921.

[132] Chas. Davisson: On the Annual and Semi-annual Seismic Periods; Roy. Soc. of London, Philosophical Transactions, Vol. 184, 1893, 1107 ff.

[133] C. G. Knott: The Physics of Earthquake Phenomena, Oxford, 1908.

[134] In Table 8 the first column indicates the region; the second, the dates; and the third, the number of shocks. The fourth column gives the month in which the annual maximum occurs when the crude figures are smoothed by the use of overlapping six-monthly means. In other words, the average for each successive six months has been placed in the middle of the period. Thus the average of January to June, inclusive, is placed between March and April, that for February to July between April and May, and so on. This method eliminates the minor fluctuations and also all periodicities having a duration of less than a year. If there were no annual periodicity the smoothing would result in practically the same figure for each month. The column marked "Amplitude" gives the range from the highest month to the lowest divided by the number of earthquakes and then corrected according to Schuster's method which is well known to mathematicians, but which is so confusing to the layman that it will not be described. Next, in the column marked "Expected Amplitude," we have the amplitude that would be expected if a series of numbers corresponding to the earthquake numbers and having a similar range were arranged in accidental order throughout the year. This also is calculated by Schuster's method in which the expected amplitude is equal to the square root of "pi" divided by the number of shocks. When the actual amplitude is four or more times the expected amplitude, the probability that there is a real periodicity in the observed phenomena becomes so great that we may regard it as practically certain. If there is no periodicity the two are equal. The last column gives the number of times by which the actual exceeds the expected amplitude, and thus is a measure of the probability that earthquakes vary systematically in a period of a year.

[135] N. F. Drake: Destructive Earthquakes in China; Bull. Seism. Soc. Am., Vol. 2, 1912, pp. 40-91, 124-133.

[136] The only other explanation that seems to have any standing is the psychological hypothesis of Montessus de Ballore as given in Les Tremblements de Terre. He attributes the apparent seasonal variation in earthquakes to the fact that in winter people are within doors, and hence notice movements of the earth much more than in summer when they are out of doors. There is a similar difference between people's habits in high latitudes and low. Undoubtedly this does have a marked effect upon the degree to which minor earthquake shocks are noticed. Nevertheless, de Ballore's contention, as well as any other psychological explanation, is completely upset by two facts: First, instrumental records show the same seasonal distribution as do records based on direct observation, and instruments certainly are not influenced by the seasons. Second, in some places, notably China, as Drake has shown, the summer rather than the winter is very decidedly the time when earthquakes are most frequent.

[137] A comparison of tropical hurricanes with earthquakes is interesting. Taking all the hurricanes recorded in August, September, and October, from 1880 to 1899, and the corresponding earthquakes in Milne's catalogue, the correlation coefficient between hurricanes and earthquakes is +0.236, with a probable error of ±0.082, the month being used as the unit. This is not a large correlation, yet when it is remembered that the hurricanes represent only a small part of the atmospheric disturbances in any given month, it suggests that with fuller data the correlation might be large.

[138] Ellsworth Huntington: The Geographic Work of Dr. M. A. Veeder; Geog. Rev., Vol. 3, March and April, 1917, Nos. 3 and 4.

[139] Frank Schlesinger: Variations of Latitude; Their Bearing upon Our Knowledge of the Interior of the Earth; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 351-358. Also Smithsonian Report for 1916, pp. 248-254.

[140] Harold Jeffreys: Causes Contributory to the Annual Variations of Latitude; Monthly Notices, Royal Astronomical Soc., Vol. 76, 1916, pp. 499-525.

[141] John Milne: British Association Reports for 1903 and 1906.

[142] C. G. Knott: The Physics of Earthquake Phenomena, Oxford, 1908.

[143] A. C. Lawson: The Mobility of the Coast Ranges of California; Univ. of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.