Page:The Scientific Monthly vol. 3.djvu/539

 WHAT Hi' A'.VOir ABOUT COMETS

��Orbital motion la carrylni the nucleus of the c

��the right.

��while they are moving out, the head is moving forward in its orbit. The nucleus obeys the gravitational attraction of the sun absolutely, so far as observation has gone, and we have no reason to suspect that it is subject to an appreciable repulsive force. The particles composing the outer regions of the head and the particles composing the tail are doubtless attracted by the gravitation of the sun and are at the same time driven away by the repulsion of the sun. What the particles will do under the action of the two opposing forces de- pends upon the ratio of these forces. If the repulsive force is vastly stronger than the attracting force the particles will travel out from the head with great and increasing speed and form a tail pointing nearly away from the sun ; that is, it will lag behind very little. If the attracting and repelling forces acting upon another group of particles are not very unequal those particles will form a second tail having considerable lag. If the repulsive force is very weak with reference to the

, ;. Fm, 7. DlAQEAM IIJ.U8THAT1NO

suns attractive force upon a third group the thbee pbincipil types ov

of particles, they will form a short tail tails of comets, orbitai mo-

,,, , J ■ 1. L- 1 mi * 1 *''"' '" cifry'DB the nucleua to

that lags very far behind. The form.s and the ittt. The sud la below.

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