Page:Popular Science Monthly Volume 7.djvu/364

350 who have studied the astounding configuration of comets. Olbers believed in a repulsive action; Bessel in a polar force; Bond, recently, on the occasion of Donati's comet, so deeply studied in America, deduced from it a simply repulsive force, and M. Roche, of Montpellier, adopted the same idea.

At first sight, it seems very strange to find the same body producing at once two opposite actions, an attraction and a repulsion. Nevertheless, if these two forces act according to different laws, they may coexist without being confounded in one single result, and may produce perfectly distinct effects. It is thus that the Newtonian attraction, which subsists between the molecules of every individual body, is by no means confounded or incorporated with the electrical or magnetic phenomena of which that same body may be the seat, or with the repulsive actions due to heat.

Now, although the most delicate observation of the celestial movements (planets and satellites) has hitherto revealed attraction alone, it is impossible at the present day to deny that the striking phenomena displayed by comets betray the existence of a quite different force, capable of driving to a distance, with incredible velocity, the most loosely attached and most attenuated particles of the matter composing them.

When a comet, arriving from the depths of the firmament, approaches the Sun, describing round him an immense ellipse almost parabolic in form, it appears to us as a spherical nebulosity more or less condensed toward its centre—that is, in the shape assumed by a body whose particles have freely taken their places under the sole influence of their mutual attractions. The sun's attraction (which at that great distance is virtually equal for all those particles), does no more than draw the comet toward it, as a whole, and in a lump as it were, without affecting its shape. But, when the distance diminishes, the parts of this sphere nearest the sun are drawn with greater force than the more distant parts diametrically opposite, and the primitive spherical figure can no longer subsist. The comet tends to grow longer and longer in the direction of the ideal line which connects it with the sun, absolutely in the same way as our globe, in its liquid portion, is drawn out into two opposite swellings familiarly known as ocean-tides. Nay, more: if the bond of mutual attraction which holds the particles of the comet together is not sufficiently powerful, it will give way; under the sun's attractive action, the comet will be decomposed, scattering its materials along its orbit, gradually transforming itself into a sort of very elongated ring of dust, like those which Schiaperelli's discovery shows to be the cause of shooting-stars when the earth happens to traverse them.

This is all that can result from the sun's attraction. But matters do not end here; and comets which have resisted for ages the destructive agency of attraction, now present quite different phenomena,