200 THE POPULAR SCIENCE MONTHLY. 



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 iden- 

 tity of four of the five elements, which exists now, and will, ac- 

 cording 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 interest- 

 ing 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 de- 

 rived for the theories of the older planets and for the most deli- 

 cate points in the theory of the moon. It would be unjust, too, 

 to forget the excellent labors of Hansen and Gylddn, 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 facil- 

 ity 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 eclip- 

 tic maps of MM. Henry deserve special mention. When these as- 

 tronomers undertook to construct the maps across the milky 

 way they were dismayed by the immensity of the work, and in- 

 voked the aid of photography. The remarkable results they ob- 

 tained have served as the point of departure for the enterprise of 

 the photographic map of the sky. The Astro-Photographic Con- 



