Since the commencement of the present century five comets have been discovered, which form, with Halley's, an interesting and remarkable group. The first of these was detected by Pons, on the 20th of July, 1812; the second by Olbers, on the 6th of March, 1815; the third by De Vico, on the 28th of February, 1846; the fourth by Brorsen, on the 20th of July, 1847; and the last by Westphal, on the 27th of June, 1852. The periods of these bodies are all nearly equal, ranging from 68 to 76 years; their eccentricities are not greatly different; the motions of all, except that of Halley's, are direct; and the distances of their aphelia are somewhat greater than Neptune's distance from the sun. Of this group, the comets of 1812 and 1846 seem worthy of special notice. The former became visible to the naked eye shortly after its discovery, and each continued visible about ten weeks. Their elements are as follows: Perihelion Passage. 1812, Sept. 1846, Mar. 15_d._ 7_h._ 5_d._ 12_h._ Long. of Perih'n. 92° 51´ 90° 31´ Long. of A. Node. 253° 33´ 77° 37´ Incl. 73° 57´ 85° 6´ Peri'n Dist. 0.7771 0.6637 Eccentricity. 0.94454 0.96224 Period. 70.68_y_ 73.715 Direction. D D Computer. Encke. Peirce. The wonderful similarity of these elements, except in the longitude of the ascending node, is at once apparent. It will also be noticed that the longitude of the _descending_ node of the latter is very nearly coincident with that of the _ascending_ node of the former. These remarkable coincidences are presented to the eye in the following diagram, where the dotted ellipse represents the orbit of the comet of 1812, and the continuous curve that of the comet of 1846. [Illustration: Fig. 3.] It is infinitely improbable that these coincidences should be accidental; they point undoubtedly to a common origin of the two bodies.