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has been rotating for centuries at high velocity. The sun, with a diameter one hundred times that of the earth (Fig.l), may throw some light on the problem. Its high temperature wholly precludes the existence of permanent magnets: hence any magnetism it may exhibit must be due to motion. Its great mass and rapid linear velocity of rotation should produce a magnetic field much stronger than that of the earth. Finally, the presence in its atmosphere of glowing gases, and the well-known effect of magnetism on light, should enable us to explore its magnetic field even at the distance of the earth. The effects of ionization, probably small in the region of high pressure beneath the photosphere and marked in the solar atmosphere, must be determined and allowed for. But with this important limitation, the sun may be used by the physicist for an experiment which can not be performed in the best equipped laboratory.

Schuster, in the lecture already cited, remarked:

The form of the corona suggests a further hypothesis which, extravagant