and will make it possible to deduce the mass of Jupiter with great precision. The planets at the lower limit of the ring may, in case their orbits are very eccentric, come very near the earth—even within a distance of 0·7—in which case it will be possible to determine their parallax accurately by observing it from two distant stations, as we do the moon. Thence we can deduce the parallax of the sun; and this is one of the best methods within our reach of obtaining this fundamental element in astronomy.
We have said that it is impossible to connect all the asteroids with the rupture of a single planet; but we can form groups of two planets whose orbits present curious resemblances which do not seem due to chance only. The most interesting group is formed of the planets 37 and 66. Their orbits are nearly equal ellipses, situated almost in the same plane, and differing only in the orientation of the major axes. This almost complete identity of four of the five elements, which exists now, and will, according to the calculations, be maintained, can not be accidental. Many facts of this kind will not be needed to illustrate to us the origin and formation of the asteroids. Maia (66) was lost for fifteen years, till it was found again by the help of M. Schulhof's calculations. There are other similar groups, and more will probably be found.
I trust that this notice will show a rich harvest of interesting facts in prospective. To speak only of acquired results, it may be recollected that Gauss composed one of his finest works for the purpose of recovering Ceres. It was in seeking for a quick way of verifying Leverrier's numerical calculations upon the great inequality of Pallas that the illustrious Cauchy wrote admirable memoirs, from which great advantages are now derived for the theories of the older planets and for the most delicate points in the theory of the moon. It would be unjust, too, to forget the excellent labors of Hansen and Gyldén, which had the same origin.
The asteroids have also been the occasion of important advances in observation. The search for them has trained observers of the first order. The purpose of following them with greater facility has led to the construction of powerful instruments, among which is the great meridian circle of the Observatory of Paris. The maps of the heavens and catalogues of stars have through them been made more nearly perfect. Among these the ecliptic maps of MM. Henry deserve special mention. When these astronomers undertook to construct the maps across the milky way they were dismayed by the immensity of the work, and invoked the aid of photography. The remarkable results they obtained have served as the point of departure for the enterprise of the photographic map of the sky. The Astro-Photographic Con-