The objections to this hypothesis are numerous. First, there is little evidence of electrolytic differentiation in the rocks. Second, the outer part of the earth's crust is a very poor conductor so that it is doubtful whether even a high degree of electrification of the surface would have much effect on the interior. Third, electrolysis due to any
such mild causes as we have here postulated must be an extremely slow process, too slow, presumably, to have any appreciable result within a month or two. Other objections join with these three in making it seem improbable that the sun's electrical activity has any direct effect upon movements of the earth's crust.
The third, or meteorological hypothesis, which makes barometric pressure the main intermediary between solar activity and earthquakes, seems at first sight almost as improbable as the thermal and electrical hypotheses. Nevertheless, it has a certain degree of observational support of a kind which is wholly lacking in the other two cases. Among the extensive writings on the periodicity of earthquakes one main fact stands out with great distinctness: earthquakes vary in number according to the season. This fact has already been shown incidentally in the table of earthquake frequency by months. If allowance is made for the fact that February is a short month, there is a regular decrease in the frequency of severe earthquakes from December and January to June. Since most of Milne's earthquakes occurred in the northern hemisphere, this means that severe earthquakes occur in winter about 20 per cent oftener than in summer.
| [TABLE 8] | ||||||
|---|---|---|---|---|---|---|
| SEASONAL MARCH OF EARTHQUAKES | ||||||
| AFTER DAVISSON AND KNOTT | ||||||
| A | B | C | D | E | F | G |
| Region | Limiting Dates | Number of Shocks | Maximum Month | Amplitude | Expected Amplitude | Ratio of Actual to Expected Amplitude |
| Northern Hemisphere | 223-1850 | 5879 | Dec. | 0.110 | 0.023 | 4.8 |
| Northern Hemisphere | 1865-1884 | 8133 | Dec. | 0.290 | 0.020 | 14.5 |
| Europe | 1865-1884 | 5499 | Dec. | 0.350 | 0.024 | 14.6 |
| Europe | 306-1843 | 1961 | Dec. | 0.220 | 0.040 | 5.5 |
| Southeast Europe | 1859-1887 | 3470 | Dec. | 0.210 | 0.030 | 7.0 |
| Vesuvius District | 1865-1883 | 513 | Dec. | 0.250 | 0.078 | 3.2 |
| Italy: | ||||||
| Old Tromometre | 1872-1887 | 61732 | Dec. | 0.490 | 0.007 | 70.0 |
| Old Tromometre | 1876-1887 | 38546 | Dec. | 0.460 | 0.009 | 49.5 |
| Normal Tromometre | 1876-1887 | 38546 | Dec. | 0.490 | 0.009 | 52.8 |
| Balkan, etc. | 1865-1884 | 624 | Dec. | 0.270 | 0.071 | 3.8 |
| Hungary, etc. | 1865-1884 | 384 | Dec. | 0.310 | 0.090 | 3.4 |
| Italy | 1865-1883 | 2350 | Dec.(Sept.) | 0.140 | 0.037 | 3.8 |
| Grecian Archip. | 1859-1881 | 3578 | Dec.-Jan. | 0.164 | 0.030 | 5.5 |
| Austria | 1865-1884 | 461 | Jan. | 0.370 | 0.083 | 4.4 |
| Switzerland, etc. | 1865-1883 | 524 | Jan. | 0.560 | 0.077 | 7.3 |
| Asia | 1865-1884 | 458 | Feb. | 0.330 | 0.083 | 4.0 |
| North America | 1865-1884 | 552 | Nov. | 0.350 | 0.075 | 4.7 |
| California | 1850-1886 | 949 | Oct. | 0.300 | 0.058 | 5.2 |
| Japan | 1878-1881 | 246 | Dec. | 0.460 | 0.113 | 4.1 |
| Japan | 1872-1880 | 367 | Dec.-Jan. | 0.256 | 0.093 | 2.8 |
| Japan | 1876-1891 | 1104 | Feb. | 0.190 | 0.053 | 3.6 |
| Japan | 1885-1889 | 2997 | Oct. | 0.080 | 0.032 | 2.5 |
| Zante | 1825-1863 | 1326 | Aug. | 0.100 | 0.049 | 2.0 |
| Italy, North of Naples | 1865-1883 | 1513 | Sept.(Nov.) | 0.210 | 0.046 | 4.6 |
| East Indies | 1873-1881 | 515 | Aug., Oct., or Dec.? | 0.071? | 0.078 | 0.9 |
| Malay Archip. | 1865-1884 | 598 | May | 0.190 | 0.072 | 2.6 |
| New Zealand | 1869-1879 | 585 | Aug.-Sept. | 0.203 | 0.073 | 2.8 |
| Chile | 1873-1881 | 212 | July | 0.480 | 0.122 | 3.9 |
| Southern Hemisphere | 1865-1884 | 751 | July | 0.370 | 0.065 | 5.7 |
| New Zealand | 1868-1890 | 641 | March, May | 0.050 | 0.070 | 0.7 |
| Chile | 1865-1883? | 316 | July, Dec. | 0.270 | 0.100 | 2.7 |
| Peru, Bolivia | 1865-1884 | 350 | July | 0.480 | 0.095 | 5.1 |
The most thorough investigation of this subject seems to have been that of Davisson.[132] His results have been worked over and amplified by Knott,[133] who has tested them by Schuster's exact mathematical methods. His results are given in Table 8.[134] Here the northern hemisphere is placed first; then come the East Indies and the Malay Archipelago lying close to the equator; and finally the southern hemisphere. In the northern hemisphere practically all the maxima come in the winter, for the month of December appears in fifteen cases out of the twenty-five in column D, while January, February, or November appears in six others. It is also noticeable that in sixteen cases out of twenty-five the ratio of the actual to the expected amplitude in column G is four or more, so that a real relationship is indicated, while the ratio falls below three only in Japan and Zante. The equatorial data, unlike those of the northern hemisphere, are indefinite, for in the East Indies no month shows a marked maximum and the expected amplitude exceeds the actual amplitude. Even in the Malay Archipelago, which shows a maximum in May, the ratio of actual to expected amplitude is only 2.6. Turning to the southern hemisphere, the winter months of that hemisphere are as strongly marked by a maximum as are the winter months of the northern
hemisphere. July or August appears in five out of six cases. Here the ratio between the actual and expected amplitudes is not so great as in the northern hemisphere. Nevertheless, it is practically four in Chile, and exceeds five in Peru and Bolivia, and in the data for the entire southern hemisphere.
The whole relationship between earthquakes and the seasons in the northern and southern hemispheres is summed up in Fig. 12 taken from Knott. The northern hemisphere shows a regular diminution in earthquake frequency from December until June, and an increase the rest of the year. In the southern hemisphere the course of events is the same so far as summer and winter are concerned, for August with its maximum comes in winter, while February with its minimum comes in summer. In the southern hemisphere the winter month of greatest seismic activity has over 100 per cent more earthquakes than the summer month of least activity. In the northern hemisphere this difference is about 80 per cent, but this smaller figure occurs partly because the northern data include certain interesting and significant regions like Japan and China where the usual conditions are reversed.[135] If equatorial regions were included in Fig. 12, they would give an almost straight line.
The connection between earthquakes and the seasons is so strong that almost no students of seismology question it, although they do not agree as to its cause. A meteorological hypothesis seems to be the only logical explanation.[136] Wherever sufficient data are available, earthquakes
appear to be most numerous when climatic conditions cause the earth's surface to be most heavily loaded or to change its load most rapidly. The main factor in the loading is apparently atmospheric pressure. This acts in two ways. First, when the continents become cold in winter the pressure increases. On an average the air at sea level presses upon the earth's surface at the rate of 14.7 pounds per square inch, or over a ton per square foot, and only a little short of thirty million tons per square mile. An average difference of one inch between the atmospheric pressure of summer and winter over ten million square miles of the continent of Asia, for example, means that the continent's load in winter is about ten million million tons heavier than in summer. Second, the changes in atmospheric pressure due to the passage of storms are relatively sharp and sudden. Hence they are probably more effective than the variations in the load from season to season. This is suggested by the rapidity with which the terrestrial response seems to follow the supposed solar cause of earthquakes. It is also suggested by the fact that violent storms are frequently followed by violent earthquakes. "Earthquake weather," as Dr. Schlesinger suggests, is a common phrase in the typhoon region of Japan, China, and the East Indies. During tropical hurricanes a change of pressure amounting to half an inch in two hours is common. On September
22, 1885, at False Point Lighthouse on the Bay of Bengal, the barometer fell about an inch in six hours, then nearly an inch and a half in not much over two hours, and finally rose fully two inches inside of two hours. A drop of two inches in barometric pressure means that a load of about two million tons is removed