Page:Popular Science Monthly Volume 18.djvu/733

Rh Mediterranean slope. The laws of the distribution of rains, which M. Belgrand announced in 1865 for the valley of the Seine, are verified by the maps, and their general application appears to be made more clear every year.

The Asteroids and Jupiter.—Dr. J. Holetschek has published, in the "Deutsche Rundschau," a summary of our present knowledge of the asteroids, or the group of bodies which revolve in orbits between those of Mars and Jupiter. Of the two hundred planets of this group which had been discovered in July, 1879, sixty-three were discovered in the United States, sixty in France, twenty-eight in Germany, seventeen in Austria, fifteen in Great Britain, eleven in Italy, five in Asia, and one in Denmark. Professor Peters, of the Clinton Observatory, has discovered more (thirty-six) than any other single observer. The orbits of one hundred and seventeen were calculated in Germany, those of forty-eight in the United States, and those of the others in Austria, France, England, Russia, and Sweden. The theory at first adopted that these bodies are the fragments resulting from the explosion of a larger planet, was contradicted by the calculations of Professor Newcomb, in 1860. D'Arrest sought to establish the fact of a connection among them by finding relations in the eccentricities of their orbits, but the elements of the planets discovered since have set this theory at rest. The idea of a collision of two bodies has also been given up. The little planets mock all attempts to combine their relations, and each asserts its individuality as an independent member of the solar system. They exhibit common features only in the limitation of their orbits, so far as the discovery of them has extended, to a particular zone, and in a corresponding limitation of their periods of revolution around the sun. Taking the earth's mean distance from the sun as unity, the halves of the major axes of their orbits may all be represented by numbers between two and four. Their periods of revolution around the sun are between four and eight times that of the earth. Great variations occur within these limits. Very perceptible and peculiar intervals exist in several cases in the mean distances of particular asteroids from the sun, which might once have been accounted for by supposing that there were planets not yet discovered which would occupy them. But as the numerous discoveries of new planets have failed to furnish the bodies sought for, and have rather rendered the gaps more obvious, it has been suggested that the vacancies are not casual, but are owing to a real natural cause. A theory has been suggested that they are occasioned by the attraction of Jupiter, and is supported by the fact that a vacancy exists at every distance from the sun at which the time of a planetary revolution would bear a definite relation to the year of Jupiter. A planet could not continue in such a position, for it would be subjected to disturbances at every conjunction with Jupiter, the effect of which would be to draw it out of its course and out of its relation with the larger planet till it found a new period of revolution not commensurable with that of Jupiter. A large gap exists between the asteroids Gerda and Sibylla, in the place which a planet making two revolutions to one of Jupiter would occupy. Gerda, having an orbit interior to this place, completes its revolutions in fifty-four days less; Sibylla, with an exterior orbit, requires a period one hundred and two days longer than that of half the year of Jupiter. Similar gaps exist at distances where planets, if there were any there, would have periods of revolution corresponding to two thirds, two fifths, three fifths, two sevenths, and three sevenths of that of Jupiter, although planets are found on either side of these spaces whose periods of revolution bear no fractional relation, or an extremely remote one, to that of Jupiter. Saturn also produces modifications in the position of the asteroids which are less noticeable on account of its greater distance and lighter mass.

The Diffusion and Softening of the Electric Light.—M. L. Clémandot, a French engineer, has invented a new arrangement for the diffusion of the electric light, which, he claims, presents considerable advantages over the opaque globes hitherto employed for that purpose. The globes operate by absorbing the light until they become luminous—a process in which a considerable