orbital revolution our globe would ever and anon come in contact with these lunar fragments; in other words, STONES _would fall occasionally to its surface, and apparently from its atmosphere_."--_Planetary System_, pp. 301, 302. We have preferred to give the views of these eminent scientists in their own language. Olbers, Biot, and Poisson, who adopted the same theory, estimated the _initial_ velocity which would be necessary in order that lunar fragments might pass the point of equal attraction, and also the _final_, or acquired velocity on reaching the earth's surface. The several determinations of the former were as follows: According to Olbers 1·570 miles a second. " Biot 1·569 " " " Laplace 1·483 " " " Poisson 1·437 " " The mean being almost exactly a mile and a half. The velocity on reaching our planet, according to Olbers, would be about six and a half miles. At the date of these calculations, however, the true velocity of aerolites had not been in any case satisfactorily determined. Since that time it has been found in numerous instances to exceed _twenty miles a second_--a velocity greater than that of the earth's orbital motion. This fact of itself would seem fatal to the theory of a lunar origin. At the meeting of the American Association for the Advancement of Science, in 1859, Dr. B. A. Gould read a paper on the supposed lunar origin of aerolites, in which the hypothesis was subjected to the test of a rigid mathematical analysis. We will not attempt even an abstract of this interesting memoir. It amounts, however, to a virtual disproof of the lunar hypothesis. THE SOLAR THEORY. The theory which ascribes a solar origin to meteorites is not of recent date, having been held by Diogenes Laertius and other ancient Greeks. Among the moderns its advocates have been much less numerous than those of the lunar hypothesis. The late Professor Charles W. Hackley, of New York, regarded shooting-stars, aerolites, and even comets, as matter projected with enormous force from the solar surface. The corona seen during total eclipses of the sun he supposed to be the emanations of this matter through the intervals of the luculi.--(See the Proceedings of the American Association for the Advancement of Science, Fourteenth Meeting, 1860.) An ingenious theory, differing in its details from that of Professor Hackley, though somewhat similar in its general features, has lately been advocated by Alexander Wilcocks, M.D., of Philadelphia, in a memoir read before the American Philosophical Society, May 20th, 1864, and published in their Proceedings. In regard to this hypothesis it seems sufficient to remark that it fails to give a satisfactory account of the annual periodicity of meteoric phenomena. CHAPTER XII. THE RINGS OF SATURN.