January 1056 =251= 804 1005 =56= February 1173 331 734 1455 126 March =1312= =335= 613 1396 =183= April 568 90 128 =1724= 148 May 170 6 1 1270 54 June =10= =0= =0= =1061= 40 July 54 0 0 1233 =35= August 191 18 40 1210 75 September 1055 209 455 =1735= 120 October =1114= =353= 716 1630 =192= November 1077 326 811 1240 112 December =940= 260 =863= 912 81 Average number 727 181 430 1322 102 In zones where the difference between the lengths of day and night of the different seasons is not very great, as in the United States, and in districts in which the southern light is observed (about latitude 40° S.), the chief minimum falls in winter: on the northern hemisphere, in December; on the southern hemisphere, in June or July. A less pronounced minimum occurring in the summer. Twice in the course of the year the earth passes through the plane of the solar equator. During these periods a minimum of solar dust trickles down upon the earth, and that period is characterized by a larger number of polar lights which is distinguished by a higher elevation of the sun above the horizon. We may expect this; for most solar dust will fall upon that portion of the earth over which the sun is highest at noon. The two maxima of March or April and of September or October, when the earth is at its greatest distance from the plane of the solar equator, are strongly marked in all the series, except in those for the polar districts Iceland and Greenland. There the auroral frequency is solely dependent upon the intensity of the twilight, so that we find a single maximum in December and the corresponding minimum in June. More recent statistics (1891-1903) indicate, however, a minimum in December. For the same reason the summer minimum in countries of high latitudes, like Sweden and Norway, is very much accentuated.