Pagina:Scientia - Vol. VII.djvu/260

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252 scientia


We also recall Prof. Backlund’s discovery that the epochs of change in the rate of acceleration of Encke’s comet coincided with times of maximum solar activity, and some reasons have lately been adduced for believing that the brightness of the same comet at different apparitions also varies with the state of the sun’s surface.

The electrical theory of the repulsion of tails explains the fact that the acceleration of some of the knots in the tail has been found to cease at a distance from the head which we may ascribe to a leakage of the charge. Also electrical excitement gives a more reasonable explanation of the glow of the gases in the tail than to suppose that they shine by actual incandescence, which can hardly occur except in the case of comets with very small perihelion distances.

To conclude, each of the three explanations of tail-repulsion seems to me to be a vera causa which we have good reason to believe is actually in operation; the only difficulty is to discriminate between the separate effects of each. This is work for the future; the rapid advance of cosmical physics in recent years gives ground for hope that the full solution is only a question of time.


Greenwich.

Andrew C. D. Crommelin
of the Royal Observatory.