93rd part of its value; and its volume-supposing the comet to be spherical, or, at all events, only changing its size, not its shape was reduced to the 813,000th part of the original volume. It appears even certain that on this second apparition the law of decrease of the diameter as compared with the diminution of the distance from the sun followed a still more rapid law of variation. Moreover, it should be remarked that, at equal distances, the comet in 1838 was somewhat less in volume than ten years before. We limit ourselves at present to the statement of the fact, as we shall give further on the explanations offered, and the difficulties which it presents in respect to the physical constitution of comets. As some connexion has been thought to exist between the changes of volume in cometary nebulosities, and the development and formation of tails, we may here remark that Encke's comet is a nebulosity of variable form—sometimes globular, sometimes oval, or more or less irregular (figs. 17, 22, 23, and 38), and that at no time has it ever exhibited a tail. SECTION I. AIGRETTES-LUMINOUS SECTORS-NUCLEAL EMISSIONS. Predominance of atmosphere in comets-Luminous sectors; emission of vaporous envelopes from the nucleus in the comets of 1835, 1858, 1860, and 1861-Formation of envelopes in Donati's comet; progressive diminution of the velocity of expansion in emissions from the nucleus. THE planets, as seen through a telescope, are bodies of regular form and definite dimensions, probably invariable, so far as we can judge from observations made in the short period of two centuries and a half that has elapsed since telescopes have been invented. A globular mass, solid or liquid, surrounded on all sides by a light and comparatively thin aëriform envelope, is perhaps, from a physical point of view, the simplest description of a planet. The comparative stability is due, on the one hand, to the preponderance of the central globe, where general phenomena are modified only at long intervals; and on the other to the trifling depth of the atmosphere, the portion of the planet the most subject to variation and internal change. In comets, we have seen, this relation is reversed, and the atmosphere or nebulous envelope constitutes the entire body, or at all events greatly preponderates. At the utmost we can only conjecture that in some comets the nucleus is solid or liquid. Certainly its volume is generally but a very insignificant portion of the entire nebulosity, even if we except |