observations, one extending through sixty years and the other through eighty-six, both of which indicate that about seventeen per cent, more thunderstorms occur in the first half of the lunar month than in the last half. Nearly 12,000 observations collected by Hazen in the United States in the year 1884 show a preponderance of thirty-three per cent, in the first half of the lunar month. Similar results have been obtained by numerous other authorities. The greatest number of thunderstorms come between new moon and the first quarter, the least number come between full moon and the last. This subject is taken up in more detail in "Popular Astronomy," 1903, X.I., 327. It has been suggested that a certain connection exists between thunderstorms and
auroras, the former being most frequent when the latter are rare, and vice versa. If such is the case, it is natural to expect to find some relation between the Moon and the aurora. Several such relations have, in fact, been found and described by H. H. Clayton.[1] He finds that the least number of auroras are visible at the time of full moon. The reason for this is obvious; but he also finds that the maximum number occurs at about the time of last quarter. Since much the greater number of auroras in the middle latitudes occur in the first half of the night, and, since there are many more observers also at that time, we should expect that during that half of the month when the Moon is not visible in the early evening, or when its light is faint, it would have little effect upon the number of auroras observed. This half of the month may be said to begin at about the fourth day after full moon. Nevertheless, the number of auroras observed at the beginning of this period is nearly three times as great as the number found at the end of it. This is exactly the reverse of what we have already found for the frequency of thunderstorms, which confirms the hypothetical relation already stated as occurring between these two forms of electrical manifestation.
Another relation between the aurora and the Moon found by Mr. Clayton depends on the lunar declination. When the Moon is farthest south in its orbit there is a marked increase in the number of northern auroras, amounting to about fifty per cent. Minor maxima also occur when the Moon is farthest north, and when it is crossing the terrestrial equator, minima being found at the four intermediate dates. Closely related to the aurora we find that certain disturbances of the Earth's magnetism occur when the Moon is at its greatest and at its least distance from us.
MacDowall counted the number of very rainy days recorded at Greenwich during the last twenty -four years. (Nature, LXIV., p. 424.) He found that the greatest
- ↑ American Journal of Science, 1898. Fourth series, V., p. 81
