Page:The Scientific Monthly vol. 3.djvu/544

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��THE SCIENTIFIC MONTHLY

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��The constant loss of materials dispelled along the tail would seem to require that comets in general grow fainter with time. This is the log- ical conclusion, and the observational evidence for it is undoubted in many of those comets which return again and again to the region of the sun. Nearly all of the Jupiter comets have a hazy, washed-out appearance. Several of them do not develop tails, as if their supply of tail materials had already been exhausted by expulsion as former tails.

Others of them develop only very short tails, and several short-period comets have entirely disappeared. To this phase of the subject we shall return.

As to the nature of the repulsive force responsible for comets' tails: It was long thought to be electrical, arising from a strong electrical field about the sun and from electric charges of the same sign on the particles composing the tail. The idea is in part purely speculative, but the giv- ing of serious consideration to it is justi- fied because of the fact that much of the light of comets seems to arise from elec- trical conditions in them. The idea may be wrong in toto, or an electric repulsive force may be one of two or more forces which are acting. It can hardly be the only force involved.

Clerk- Maxwell half a century ago, from pure theory, and Lebedew and Nichols and Hull some fifteen years ago, from experi- mental evidence admitting of no doubt, showed that when light energy falls upon a surface it presses against that surface; very feebly it is true, but it will cause the body pressed upon to move if that body is not too massive. In this respect light- pressure repulsion and electric repulsion should act much alike. These repulsions are effective in proportion to the surface Fio. 11. succBssivH Positions «^reas of the bodies acted upon, whereas OF THB Inner End op a dbtachbd gravitation pulls those bodies with a force

Tail of Halle y's Comet, Joni,. i x i i • -^-r .1

4_8^ 1910. proportional to their masses. Now the sur-

face of a body is proportional to the square of its dimensions, whereas gravity acts in proportion to the cube of its dimensions. The smaller a body is the more surface it has in proportion to

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