Page:A Voyage in Space (1913).djvu/61

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teresting because it shows there is another force besides gravity which affects them. The head of the comet feels only the pull of the Sun, which is very strong when it is near the Sun and very slight when far away; but the tail of the comet feels another force altogether, and behaves in a totally different way. Instead of following the comet all the way like the smoke of an engine, it points directly away

Voyage in Space page041.jpg

Various Positions of a Comet's Tail.

from the Sun. That is due to the fact that besides gravity that pulls the head towards the Sun, there is another force away from the Sun; it is as though the Sun was blowing the tail of the comet away from the head. Of course there is no actual blowing as one blows with one's breath; it may be electrical repulsion; or it may be light pressure the fierce light of the Sun acting as a blast would act and blowing the tail out.

We can see this action of the Sun in blowing away