parent atmosphere, was globular. The figure of the luminous envelope the doctor states to have been that of an inverted hollow cone, terminating at its vertex in a hollow cup of nearly a hemispherical construc→ tion. The whole of this envelope he considers to have been very thin. A rotary motion of the comet, he thinks, would account for the variations observed in the length of the opposite branches which enclosed the tail. In comparing the comet of 1807 with the present one, Dr. H. observes, The first of these, in its ap proach to the Sun, came within 61 millions of miles of it; and its tail, when longest, covered an extent of 9 millions. The present one in its perihelion did not come so near the Sun by 36 millions of miles, and nevertheless acquired a tail 91 millions of miles longer than the former. The difference of their distances from the Earth when these measures were taken was only about 2 millions of miles.' From this circumstance he conjectures that the comet of 1807 was in a state of greater consolidation, either from having previously approached the Sun or some of the fixed stars, which there is reason to believe are of the same nature as the Sun. He then concludes the paper with some conjectures relative to the comparative ages of the two comets, and some remarks on the increase and original formation of comets; for which we must refer the curious reader to the paper itself, merely ob serving that we regard them as conjectures only. Besides these two celebrated and beautiful visitants which have recently traversed those parts of the solar system, another small telescope comet was discovered by M. Olbers in the early part of March 1815. This comet was so small and distant as to be seen only by astronomers; and respecting which Delambre, in his report of the labours of the Class of Mathematical and Physical Sciences of the Royal Institute of France during the year 1815, makes the following re marks: The astronomers of Paris, disappointed by the weather, could make but a small number of observations; from which, however, M. Nicollet deduced a parabolic orbit. Foreign astronomers, sooner aware of the appearance of the comet, and less distracted by circumstances, were able to follow it for a greater length of time, and with more assiduity. From these more numerous observations, they have deduced an elliptic orbit; and it is remarkable, that the greater axis of this orbit is less than that of the orbit of Uranus, and even less than that of the comet of 1759, whose periodic revolution is between 75 and 76 years. We may therefore expect the return of this last in about 73 years. This expectation at least is justified by the results of several eminent astronomers, who have separately arrived at this conclusion, with a correspondence very uncommon in a problem which can never be solved with certainty after a single appearance. This agreement is evident from the following elements of the two elliptical orbits. Elements according to MM. Lindenau and Nicholai. The Time is that at Sceberg. for the nature of comets, especially for their tails, which assume such a variety of appearances. But as these are more speculative than either useful or certain, we shall not enter into many particulars respecting them. Newton thought the tails consisted of some kind of vapour arising constantly from the head or nucleus of the comet. The ascent of these vapours into the tail he supposed to be occasioned by the rarefaction of its atmosphere on its approach to the Sun. Euler conceived that a great affinity existed between the tails of comets, the zodiacal light, and the aurora borealis; and that the common cause of all these was the actions of light on the atmospheres of comets, the Sun, and the earth. Dr. Hamilton, in the second of his Philosophical Essays, shows that the tails of comets, the aurora borealis, and the electric fluid, agree remarkably, both in appearance and in such properties as can be observed re lative to each of them; from which he concludes that they are substances of the same nature. 6 Ánother ingenious hypothesis advanced to account for the nature of these heavenly bodies is contained in a letter from M. De Luc to M. Bode, the Astronomer Royal, at Berlin. This philosopher lays it down as a principle relative to such bodies as are capable of becoming luminous, that the light which escapes from them had entered into their composition as an ingredient, and is evolved by chemical decomposition.' This author regards the vapour which constitutes the tails of comets to be of a phosphoric nature, and the action of the solar rays he considers as the chemical cause which regulates either the production or the decomposition of these vapours, or both, and gives rise to these luminous effects. Though an approach to the Sun may produce a more rapid decomposition of these vapours, M. De Luc asserts that it cannot always produce the same effect, either on different comets, or on the same comet at different times, unless the production of these vapours proceed with the same intensity, but which we do not know to be the case. He thinks, that if these vapours rise to a considerable height above a comet before they are decomposed, when the comet approaches its perihelion, and they become luminous, it may appear like a nebulosity only, without our being able to perceive any nucleus; and that when we can perceive a determinate disc, it is probably owing to the regularity in the extent of the lucid vapours, without the body of the comet being really visible. The tails of comets have always been observed to be on that side of them which is farthest from the Sun, and to have their convexities turned towards the side to which they are moving; and the former of these circumstances is accounted for by this writer, by supposing these tails to consist of a fluid similar to the electric fluid, proceeding from the comet, and rendered luminous by decomposition in its course. The cause of the curvature he supposes to arise from the particles of the fluid of which they are composed partaking of the same projectile motion as the comet at the moment they are emitted from it; and having attained a considerable distance from the body of the comet, and continuing to move with the same velocity, but in a larger orbit, their angular motion will be diminished, and the curvature be a natural consequence. The length of the tail, according to this hypothesis, will be greater as the production of the fluid is more abundant and its decomposition more slowly effected; and this writer thinks that these circumstances may be subject to such variations, even in the same comet, as to preclude its identity from being ascertained. This, however, can only apply to the difference in the appearance of the same comet on its return; for if the elements of its orbit be the same, or nearly the same, in both cases, as we have already observed, its identity ought to be inferred, though the appearance may be somewhat different. [To be continued.] The Naturalist's Diary For OCTOBER 1818. Farewel ye wild hills, scattered o'er with spring! * * * * * * Farewel the walk along the woodland vale! That spring's green hours to fancy's children bore; On slumber's light leaf, by the murmuring shore. Fly the dear dreams of spring's delightful reign; And rude winds waste the glories of her train. Yet AUTUMN yields her joys of humbler kind; LANGHORNE, THE groves now lose their leafy honours; but, before they are entirely tarnished, an adventitious beauty, arising from that gradual decay which loosens the withering leaf, gilds the autumnal landscape with a temporary splendour, superior to the verdure of spring, or the luxuriance of summer. The infinitely various and ever-changing hues of the leaves at this season, melting into every soft gradation of tint and shades, will long continue to engage the imitation of the painter, and the contemplation of the poet and the philosopher. The scenes, presented to our |