place. The daily oscillation is itself variable in a systematic manner, not only with the progress of the year, but with that of the lunar month. The daily oscillation also varies at times in a sudden and irregular manner. The needle has been exhibiting for several weeks the most perfect uniformity of oscillation. Day after day the careful observation of the needle's progress has revealed a steady swaying to and fro, such as may be seen in the masts of a stately ship at anchor on the scarce-heaving breast of ocean. Suddenly a change is noted; irregular movements become perceptible which are totally distinct from the regular periodic oscillations. A magnetic storm is in progress, and its progress does not affect only the place of observation, but widely-extended regions of the Earth; and in some well-authenticated instances, these magnetic vibrations thrill in one moment the whole frame of the Earth.

Lamont, of Munich, was the first to announce that these magnetic disturbances attain a maximum of frequency in periods of about ten years. This was in 1850. Two years later General Sabine and (independently) Professors Wolf and Gautier noted the coincidence of this period with that of the solar spots. Of course, mere coincidence in duration was not the sole circumstance on which they based this conclusion. was the coincidence of maximum of spot-frequency with maximum of magnetic disturbance, and of minimum with minimum, which enabled them to assert the true correspondence of the two periods.

It

Wolf subsequently proved that the period of magnetic disturbances has the length of 11.11 years, which he had assigned as the correct value of the solar spot period.

A relation so strange might well excite grave doubts. Coincidences have so often misled men of science, and indeed it is so certain in the very nature of things that misleading coincidences must occur, that physicists were justified in requiring further evidence. Such evidence fortunately was not wanting. Independently of the continuance of observation and the close correspondence which has been observed during the past score of years between Sun-spots and magnetic phenomena, an occurrence of a very interesting nature served in 1859 to place beyond all possibility of question the influence which solar action exerts upon the Earth's magnetism. I have so often described the occurrence in my own words that I think it well in the present instance to give the words of the two telescopists by whom it was independently observed, especially as the details of the observation have recently been called in question.

On September 1, 1859, Mr. Carrington was engaged in taking his customary observation of the forms and positions of the solar spots. I had secured diagrams,' he says, of all the groups and detached spots, and was engaged at the time in counting from a chronometer and recording the contacts of the spots with the cross-wires used in the observation, when within the area of the great north group (the size of which had

previously excited general remark) two patches of intensely bright and white light broke out' in the

middle of the group. My first impression was,' he adds, that by some chance a ray of light had penetrated a hole in the screen attached to the object glass, by which the general image is thrown into shade, for the brilliancy was fully equal to that of direct sunlight; but by at once interrupting the current observation, and causing the image to move by turning the right ascension handle, I saw I was an unprepared witness of a very different affair. I thereupon noted down the time by the chronometer, and seeing the outbreak to be very rapidly on the increase, and being somewhat flurried by the surprise, I hastily ran to call some one to witness the exhibition with me, and on returning within 60 seconds, was mortified to find that it was already much changed and enfeebled. Very shortly afterwards the last trace was gone, and although I maintained a strict watch for nearly an hour no recurrence took place. The spots had travelled considerably from their first position, and vanished as two rapidly fading dots of white light. The instant of the first outburst was not fifteen seconds different from 11h. 18m. Greenwich mean time, and 11h. 23m. was taken for the time of disappearance. In this interval of five minutes, the two spots traversed a space of about 35,000 miles, It was impossible on first witnessing an appearance so similar to a sudden conflagration, not to expect a considerable result in the way of alteration of the details

of the group in which it occurred; and I was certainly surprised, on referring to the sketch which I had carefully and satisfactorily (and I may add, fortunately) finished before the occurrence, at finding myself unable to recognise any change whatever as having taken place. The impression left upon me is that the phenomenon took place at an elevation considerably above the general surface of the Sun, and accordingly altogether above and over the great group in which it was seen projected. Both in figure and position the patches of light seemed entirely independent of the configuration of the great spot, and of its parts, whether nucleus or umbra.'

Mr. Hodgson's account (written before he had exchanged any information' with Mr. Carrington) runs as follows:- While observing a group of spots on September 1, I was suddenly surprised at the appearance of a very brilliant star of light, much brighter than the Sun's surface, most dazzling to the protected eye, illuminating the upper edges of the adjacent spots and streaks, not unlike in effect the edging of the clouds at sunset; the rays extended in all directions; and the centre might be compared to the dazzling brilliancy of the bright star Alpha Lyra when seen in a large telescope with low power. It lasted for some five minutes, and disappeared instantaneously, about 11h. 25m. A.M.' It seems probable that whereas two spots were seen by Mr. Carrington, who observed the solar image projected on a screen, these were blended, owing to their extreme brilliancy, into the semblance

of a single spot when observed in the telescope itself by Mr. Hodgson.

At the moment when the Sun was thus disturbed, the magnetic instruments at Kew exhibited those signs which indicate the occurrence of a magnetic storm. It was found,' says Dr. Balfour Stewart, that a magnetic disturbance had broken out at the very moment when this singular appearance had been observed.' But this was not all. A magnetic storm never rages without accompanying signs of disturbance. Auroras in both hemispheres, and the interruption of magnetic communication all over the Earth, are the signs of a great magnetic storm. Both these evidences of great disturbance were afforded during the hours which followed the solar outbreak witnessed by Carrington and Hodgson. Vivid auroras were seen not only in both hemispheres, but in latitudes where auroras are very seldom witnessed. Even in Cuba the sky was illuminated by the auroral radiance. Strong earth-currents were observed along telegraphic lines, and these currents continually changed their direction, while all the time the magnetic needles in fixed observatories were kept markedly on one side of their normal position. By degrees,' says Sir John Herschel,* accounts began to pour in of great auroras seen

6

I have been careful, it will be noticed, to quote the words of many authorities in dealing with this matter. The account I gave in my Other Worlds than Ours has been cavilled at by the Cavilian Professor of Astronomy as if it had been based on imagination; so that I have thought it well to re-examine the records and statements from which that account was really taken (though by the aid of memory instead of