SECTION IV.

HYPOTHESIS OF AN APPARENT REPULSION.

Views of Newton on the formation of the tails of comets-Action of heat and rarefac

tion of the cometary matter-The ethereal medium, losing its specific weight, rises opposite the sun, and carries with it the matter of the tail-Objections which have been made to the hypothesis of a resisting and ponderable medium.

NEWTON, in order to explain the formation of the tails of comets, had recourse to no other causes than the ordinary action of the calorific rays on the one hand, and that of gravitation on the other. But, although he does not introduce any new force, he is obliged to suppose that the comet during the whole time that its tail is developing is traversing a medium subject to the force of gravitation and tending towards the sun. Newton thus explains the theory:

The tail is composed of vapours, that is to say, of the lightest parts of the atmosphere of a comet. These vapours are rarefied by the action of the solar heat, and in their turn heat the surrounding ether. Thus, the medium which surrounds the comet becomes rarefied; it consequently loses its specific weight, and instead of tending with the same energy towards the sun, it continues to rise in the same manner as layers of air heated at the surface of the soil rise in virtue of the principle of Archimedes. In rising it carries with it particles of cometary matter, which by their ascension produce the tail, rendered visible by the reflexion of light proceeding

from the sun. In this manner smoke ascends in a chimney by the impulsion of the air in which it is suspended; this air is rarefied by the heat; it ascends because its gravity or specific weight has become less, and it draws along in its ascent columns of smoke. The ascension of cometary vapours further arises from the fact that they revolve about the sun, and for this reason have a tendency to fly from it. The atmosphere of the sun or matter of the heavens is at rest or turns slowly, having received its movement of rotation from the sun. Such are the causes which determine the ascent of cometary tails in the vicinity of the sun where the orbits are much curved, and where the comet, plunged in a dense and consequently heavier atmosphere, emits a longer tail.

This theory, which had been in the first place vaguely formulated by Riccioli, and then by Hooke (the latter, we may remember, inclines rather to the doctrine of a repulsive force), was adopted by different astronomers of the eighteenth century, Boscovich, Gregory, Pingré, Delambre, Lalande. Gregory, however, was not contented with the cause assigned by Newton for the ascent of cometary tails; he believed also in an active impulsion in addition to an apparent repulsive force. His system is a combination of the two systems we have just described.

Various objections of a serious kind have been urged against the theory of Newton. The existence of a resisting medium, gravitating towards the sun, of a solar atmosphere, in fact, would necessarily be limited to within a certain distance of the sun himself. Laplace has proved that for such an atmosphere to subsist it must be animated by a movement of rotation about the sun's axis, and that it could not extend beyond the distance at which the centrifugal force arising from that movement would become equal to the force of gravitation. In the plane of the solar equator the limit is seventeen hundredths of the mean distance of the earth; it corresponds to the radius

of the orbit of a planet whose revolution would be equal in duration to the solar rotation, which is effected in twenty-five days and a half. Now, comets, before attaining such proximity to the sun, are provided with tails; and considerable tails have been exhibited by comets whose perihelion distance has even exceeded the radius of the terrestrial orbit, which is nearly six times as great as the extreme possible limit of the solar atmosphere.

Besides, this ponderable medium would be a resisting medium as well. In addition to the disturbing action that this resistance would exercise upon the head of the comet and likewise upon its orbit, it would act with much more intensity upon the tail of the comet, on account of its extreme rarity. Before the perihelion passage, in the first part of the comet's movement, the curvature and the drifting back of the tail would be easily explained by this resistance; but, after the perihelion passage, the tail continues to keep the same position relatively to the radius vector joining the nucleus to the sun, so that the comet appears to move its tail to a position in advance of itself, a phenomenon incompatible with the hypothesis of a resisting medium. The medium of which we speak has likewise been assimilated to the zodiacal light; and Mairan, who has thus explained the terrestrial Aurora Borealis, finds in this light the cause and origin of cometary tails. But the preceding objections and others, which would take too long to repeat here, have been justly opposed to this new theory.

SECTION V.

THEORY OF OLBERS AND BESSEL.

Hypothesis of an electric or magnetic action in the formation of tails-Repulsive action of the sun upon the cometary matter, and of the nucleus upon the nebulosity -Views of Sir John Herschel and M. Liais-Theory of Bessel-Oscillations of luminous sectors-Magnetic polar force.

WHETHER the cause which determines the production of cometary tails and their development, at once so immense and so rapid, be a force sui generis, or only an apparent force, it is none the less true that it has all the features of a repulsive action or force. Heat, the impulsion of the solar rays, gravitation, have all been variously combined in order to furnish the desired explanation; it evidently remained to try the intervention of the electric and magnetic forces.

From this point of view Olbers, Herschel, and Bessel have in turn applied themselves to the problem. We will give a brief analysis of the opinions held by these illustrious

astronomers.

The comet of 1811 first drew the attention of Olbers to the subject. This astronomer,' says M. Roche, 'attributes to the proximity of the comet and the sun a development of electricity in both these bodies; hence arises a repulsive action of the sun and another repulsive action of the comet upon the nebulosity which surrounds it.' By the first of these forces Olbers has

explained the formation and development of tails; by the second he has accounted for the formation of the luminous sectors or plumes of the comet, and also the successive envelopes similar to those which were observed in Donati's Biot has given his adhesion to this theory.

comet.

Sir John Herschel's view is nearly the same. It is not improbable, he observes, that the sun is constantly charged with positive electricity; that as the comet draws near the sun and its substance becomes vaporised the separation of the two electricities takes place, the nucleus becoining negative and the tail positive. The electricity of the sun would direct the movement of the tail, just as an electrified body acts upon a non-conducting body electrified by influence.*

* In his Outlines of Astronomy Sir J. Herschel is less explicit in regard to the physical nature of the force which produces the tails, and he does not refer to electricity. But, speaking of the curious phenomena which were observed to take place in the head of Halley's comet during its apparition of 1835, he proceeds :

'Reflecting on these phenomena, and carefully considering the evidence afforded by the numerous and elaborately executed drawings which have been placed on record by observers, it seems impossible to avoid the following conclusions:

'1st. That the matter of the nucleus of a comet is powerfully excited and dilated into a vaporous state by the action of the sun's rays, escaping in streams and jets at those points of its surface which oppose the least resistance, and in all probability throwing that surface or the nucleus itself into irregular motions by its reaction in the act of so escaping, and thus altering its direction.

'2ndly. That this process chiefly takes place in that portion of the nucleus which is turned towards the sun, the vapour escaping freely in that direction.

'3rdly. That when so emitted it is prevented from proceeding in the direction originally impressed upon it by some force directed from the sun, drifting it back and carrying it out to vast distances behind the nucleus, forming the tail, or so much of the tail as can be considered as consisting of material substance.

'4thly. That this force, whatever its nature, acts unequally on the materials of the comet, the greater portion remaining unvaporised; and a considerable part of the vapour actually produced remaining in its neighbourhood, forming the head and coma.

5thly. That the force thus acting upon the materials of the tail cannot possibly be identical with the ordinary gravitation of matter, being centrifugal or