The association between auroral phenomena and those of terrestrial magnetism has long been placed beyond a doubt. Wargentin in 1750 first established the fact, which had been previously noted, however, by Halley and Celsius. But the extension of the relation to phenomena occurring outside the earth--very far away from the earth--belongs to recent times. The first point to be noticed, as showing that the aurora depends partly on extra-terrestrial circumstance, is the fact that the frequency of its appearance varies greatly from time to time. It is said that the aurora was hardly ever seen in England during the Seventeenth Century, although the northern magnetic pole was then much nearer to England than it is at present Halley states that before the great aurora of 1716 none had been seen (or at least recorded) in England for more than eighty years, and no remarkable aurora since 1574. In the records of the Paris Academy of Sciences no aurora is mentioned between 1666 and 1716. At Berlin one was recorded in 1707 as a very unusual phenomenon; and the one seen at Bologna in 1723 was described as the first which had ever been seen there. Celsius, who described in 1733 no less than three hundred and sixteen observations of the aurora in Sweden between 1706 and 1732, states that the oldest inhabitants of Upsala considered the phenomenon as a great rarity before 1716. Anderson of Hamburg states that in Iceland the frequent occurrence of auroras between 1716 and 1732 was regarded with great astonishment. In the Sixteenth Century, however, they had been frequent. Here then we seem to find the evidence of some cause external to the earth as producing auroras, or at least as tending to make their occurrence more or less frequent. The earth has remained to all appearance unchanged in general respects during the last three centuries, yet in the Sixteenth her magnetic poles have been frequently surrounded by auroral streamers; during the Seventeenth these streamers have been seldom seen; during the last two-thirds of the Seventeenth Century auroras have again been frequent; and during the Eighteenth Century they have occurred sometimes frequently during several years in succession, at others very seldom. Connected as auroras are with the phenomena of terrestrial magnetism, we may expect to find some help in our inquiry from the study of these phenomena. Now it appears certain that magnetic phenomena are partly influenced by changes in the sun’s condition. We may well believe that they are in the main due to the sun’s ordinary action, but the peculiarities which affect them seem to depend on _changes_ in the sun’s action. Many of my readers will doubtless remember the auroras of May 13, 1869, and October 24, 1870, both of which occurred when the sun’s surface was marked by many spots, and both of which were accompanied by remarkable disturbance of the earth’s magnetism.