about three million miles nearer the sun in the winter than in the summer, and the number of particles reaching it will vary, like the intensity of light, inversely as the square of the distance. These coincidences are certainly very striking, and they have a cumulative force. If we accept the theory, it would appear that we ought to congratulate ourselves that the inclination of the sun’s equator is so slight, for as things stand the earth is never directly over the most active regions of the sun-spots, and consequently never suffers from the maximum bombardment of charged particles of which the sun is capable. Incessant auroral displays, with their undulating draperies, flitting colors, and marching columns might not be objectionable from the point of view of picturesqueness, but one magnetic storm of extreme intensity following closely upon the heels of another, for months on end, crazing the magnetic needle and continually putting the telegraph and cable lines out of commission, to say nothing of their effect upon “wireless telegraphy”, would hardly add to the charms of terrestrial existence. [Illustration: Auroral arches seen in Scandinavia] One or two other curious points in connection with Arrhenius’ hypothesis may be mentioned. First, the number of auroræ, according to his explanation, ought to be greatest in the daytime, when the face of the earth on the sunward side is directly exposed to the atomic bombardment. Of course visual observation can give us no information about this, since the light of the Aurora is never sufficiently intense to be visible in the presence of daylight, but the records of the magnetic observatories can be, and have been, appealed to for information, and they indicate that the facts actually accord with the theory. Behind the veil of sunlight in the middle of the afternoon, there is good reason to believe, auroral exhibitions often take place which would eclipse in magnificence those seen at night if we could behold them. Observation shows, too, that auroræ are more frequent before than after midnight, which is just what we should expect if they originate in the way that Arrhenius supposes. Second, the theory offers an explanation of the alleged fact that the formation of clouds in the upper air is more frequent in years when auroræ are most abundant, because clouds are the result of the condensation of moisture upon floating particles in the atmosphere (in an absolutely dustless atmosphere there would be no clouds), and it has been proved that negative ions like those supposed to come from the sun play a master part in the phenomena of cloud formation.