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bodies; however, Professor Encke not only proved their identity, but determined the circumstances of the comet's motion. Its reappearance in the years 1825, 1828, and 1832 accorded with the orbit assigned by M. Encke, who thus established the length of its period to be 1207 days, nearly. This comet is very small, of feeble light, and invisible to the naked eye, except under very
favourable circumstances, and in particular positions; it has no tail, it revolves in an ellipse of great excentricity inclined at an angle of 13° 22' to the plane of the ecliptic, and is subject to considerable perturbations from the attraction of the planets. It has already been mentioned, that among the many perturbations to which the planets are liable, their mean motions, and, therefore, the major axes of their orbits, experience no change; while, on the contrary, the mean motion of the moon is accelerated from age to age, a circumstance at first attributed to the resistance of an etherial medium pervading space, but subsequently proved to arise from the secular diminution of the excentricity of the terrestrial orbit. Although the resistance of such a medium has not hitherto been perceived in the motions of such dense bodies as the planets and satellites, its effects on the revolutions of the two small periodic comets hardly leave a doubt its existence. From the numerous observations that have been made on each return of the comet of the short period, the elements have been computed with great accuracy on the hypothesis of its moving in vacuo; its perturbations occasioned by the disturbing action of the planets have been determined ; and after every thing that could influence its motion had been duly considered, M. Encke found that an acceleration of about two days on each revolution has taken place in its mean motion, precisely similar to that which would be occasioned by the resistance of an etherial medium; and as it cannot be attributed to a cause like that which produces the acceleration of the moon, it must be concluded that the celestial bodies do not perform their revolutions in an absolute void, and that although the medium be too rare to have a sensible effect on the masses of the planets and satellites, it nevertheless has a considerable influence on so rare a body as a comet. Contradictory as it may seem, that the motion of a body should be accelerated by the resistance of an etherial medium, the truth becomes evident if it be considered that both planets and comets are retained in their orbits by two forces which exactly balance one another ; namely, the centrifugal force producing the velocity in the tangent, and the attraction of the gravitating force directed to the centre of the sun. If one of these forces
be diminished by any cause, the other will be proportionally increased. Now, the necessary effect of a resisting medium is to diminish the tangential velocity, so that the balance is destroyed, gravity preponderates, the body descends towards the sun till equilibrium is again restored between the two forces; and as it then describes a smaller orbit, it moves with increased velocity. Thus, the resistance of an etherial medium actually accelerates the motion of a body, but as the resisting force is confined to the plane of the orbit it has no influence whatever on the inclination of the orbit, or on the place of the nodes. The other comet belonging to our system, which returns to its perihelion after a period of 6 years, has been accelerated in its motion by a whole day during its last revolution, which puts the existence of ether beyond a doubt, and forms a strong presumption in corroboration of the undulating theory of light. The comet in question was discovered by M. Biela at Johannisberg on the 27th of February, 1826, and ten days afterwards it was seen by M. Gambart at Marseilles, who computed its parabolic elements, and found that they agreed with those of the comets which had appeared in the years 1789 and 1795, whence he concluded them to be the same body moving in an ellipse, and accomplishing its revolution in 2460 days. The perturba
tions of this comet were computed by M. Damoiseau, who predicted that it would cross the plane of the ecliptic on the 29th of October, 1832, a little before midnight, at a point nearly 18484 miles within the earth's orbit; and as M. Olbers, of Bremen, in 1805, had determined the radius of the comet's head to be about 21136 miles, it was evident that its nebulosity would envelope a portion of the earth's orbit, a circumstance which caused great alarm in France, and not altogether without reason, for if any disturbing cause had delayed the arrival of the comet for one month, the earth must have passed through its head. M. Arago dispelled their fears by the excellent treatise on comets which appeared in the Annuaire of 1832, where he proves that, as the earth would never be nearer the comet than 24800000 British leagues, there could be no danger of collision.
If a comet were to impinge on the earth, so as to destroy its centrifugal force, it would fall to the sun in 64; days. What the earth's primitive velocity may have been it is impossible to say; therefore a comet may have given it a shock without changing the axis of rotation, but only destroying part of its 'tangential velocity, so as to diminish the size of the orbit, a thing by no means impossible, though highly improbable; at all events, there no proof that such has been
the case; and it is manifest that the axis of the earth's rotation has not been changed, because, as there is no resistance, the libration would to this day be evident in the variation it must have occasioned in the terrestrial latitudes. Supposing the nucleus of a comet to have a diameter only equal to the fourth part of that of the earth, and that its perihelion is nearer to the sun than we are ourselves, its orbit being otherwise unknown, M. Arago has computed that the probability of the earth receiving a shock from it is only one in 281 millions, and that the chance of our coming in contact with its nebulosity is about ten or twelve times greater. But in general comets are so rare, that it is likely they would not do much harm if they were to impinge; and even then the mischief would probably be local, and the equilibrium soon restored, provided there was only a gaseous or very small nucleus. It is, however, more probable that the earth would only be deflected a little from its course by the approach of a comet, without being touched by it. The comets that seem to have come nearest to the earth were that of the year 837, which remained four days within less than 1240000 leagues from our orbit; that of 1770, which approached within about six times the distance of the moon. The celebrated comet of 1680 also came very near to us; and