In ascertaining the relation between sunspots and earthquakes it would be well if we could employ the strict method of correlation coefficients. This, however, is impossible for the entire century, for the record is by no means homogeneous. The earlier decades are represented by only about one-fourth as many earthquakes as the later ones, a condition which is presumably due to lack of information. This makes no difference with the method
employed in Table 7, since years with many and few sunspots are distributed almost equally throughout the entire nineteenth century, but it renders the method of correlation coefficients inapplicable. During the period from 1850 onward the record is much more nearly homogeneous, though not completely so. Even in these later decades, however, allowance must be made for the fact that there are more earthquakes in winter than in summer, the average number per month for the fifty years being as follows:
| Jan. 2.8 | May 2.4 | Sept. 2.5 |
| Feb. 2.4 | June 2.3 | Oct. 2.6 |
| Mar. 2.5 | July 2.4 | Nov. 2.7 |
| Apr. 2.4 | Aug. 2.4 | Dec. 2.8 |
The correlation coefficient between the departures from these monthly averages and the corresponding departures from the monthly averages of the sunspots for the same period, 1850-1899, are as follows:
Sunspots and earthquakes of same month: +0.042, or 1.5 times the probable error.
Sunspots of a given month and earthquakes of that month and the next: +0.084, or 3.1 times the probable error.
Sunspots of three consecutive months and earthquakes of three consecutive months allowing a lag of one month, i.e., sunspots of January, February, and March compared with earthquakes of February, March, and April; sunspots of February, March, and April with earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the probable error.
These coefficients are all small, but the number of individual cases, 600 months, is so large that the probable error is greatly reduced, being only ±0.027 or ±0.028. Moreover, the nature of our data is such that even if
there is a strong connection between solar changes and earth movements, we should not expect a large correlation coefficient. In the first place, as already mentioned, the earthquake data are not strictly homogeneous. Second, an average of about two and one-half strong earthquakes per month is at best only a most imperfect indication of the actual movement of the earth's crust. Third, the sunspots are only a partial and imperfect measure of the activity of the sun's atmosphere. Fourth, the relation between solar activity and earthquakes is almost certainly indirect. In view of all these conditions, the regularity of Table 7 and the fact that the most important correlation coefficient rises to more than four times the probable error makes it almost certain that the solar and terrestrial phenomena are really connected.
We are now confronted by the perplexing question of how this connection can take place. Thus far only three possibilities present themselves, and each is open to objections. The chief agencies concerned in these three possibilities are heat, electricity, and atmospheric pressure. Heat may be dismissed very briefly. We have seen that the earth's surface becomes relatively cool when the sun is active. Theoretically even the slightest change in the temperature of the earth's surface must influence the thermal gradient far into the interior and hence cause a change of volume which might cause movements of the crust. Practically the heat of the surface ceases to be of appreciable importance at a depth of perhaps twenty feet, and even at that depth it does not act quickly enough to cause the relatively prompt response which seems to be characteristic of earthquakes in respect to the sun.
The second possibility is based on the relationship between solar and terrestrial electricity. When the sun is active the earth's atmospheric electrical potential is
subject to slight variations. It is well known that when two opposing points of an ionized solution are oppositely charged electrically, a current passes through the liquid and sets up electrolysis whereby there is a segregation of materials, and a consequent change in the volume of the parts near the respective electrical poles. The same process takes place, although less freely, in a hot mass such as forms the interior of the earth. The question arises whether internal electrical currents may not pass between the two oppositely charged poles of the earth, or even between the great continental masses and the regions of heavier rock which underlie the oceans. Could this lead to electrolysis, hence to differentiation in volume, and thus to movements of the earth's crust? Could the results vary in harmony with the sun? Bowie[131] has shown that numerous measurements of the strength and direction of the earth's gravitative pull are explicable only on the assumption that the upheaval of a continent or a mountain range is due in part not merely to pressure, or even to flowage of the rocks beneath the crust, but also to an actual change in volume whereby the rocks beneath the continent attain relatively great volume and those under the oceans a small volume in proportion to their weight. The query arises whether this change of volume may be related to electrical currents at some depth below the earth's surface.