The existence of a trans-Neptunian planet has been suspected by many astronomers. In November 1879 the first idea of its existence was thrown out by Flammarion in his ‘Popular Astronomy.’ Flammarion noticed that all the periodical comets in the Solar System have their aphelion near the orbit of a planet. Thus Jupiter owns about eighteen comets; Saturn owns one, and probably two; Uranus two or three; and Neptune six. The third comet of 1862, however, along with the August meteors, goes farther out than the orbit of Neptune. Accordingly, Flammarion suggested the existence of a great planet, assigning it a period of 330 years and a distance of 4000 millions of miles. Two independent investigators, _David Peck Todd_ (born 1855) in America and _George Forbes_ in Scotland, have since undertaken to find the planet. Todd, utilising the “residual perturbations” of Uranus, assigned a period of 375 years for his planet. Forbes, on the other hand, working from the comet theory, stated his belief in the existence of two planets with periods of 1000 and 5000 years respectively. In October 1901 he computed the position of the new planet on the celestial sphere, fixing its position in the constellation Libra, and computing its size to be greater than Jupiter. A search was made by means of photography, in 1902, but without success. Nevertheless, astronomers are pretty confident of the existence of one or more trans-Neptunian planets. Lowell is very definite on this subject when he says in regard to meteor groups, “The Perseids and the Lyrids go out to meet the unknown planet, which circles at a distance of about forty-five astronomical units from the Sun. It may seem strange to speak thus confidently of what no mortal eye has seen, but the finger of the sign-board of phenomena points so clearly as to justify the definite article. The eye of analysis has already suspected the invisible.” CHAPTER VII. COMETS. At the time of Herschel the ancient superstitions in regard to comets had to a great extent vanished, thanks mainly to the return of Halley’s comet in 1758. Yet, although comets had ceased to be objects of terror, no explanation or rational theory of their nature was put forward until the appearance of the great comet of 1811. This comet was visible from March 26, 1811, to August 17, 1812, a period of 510 days. It was one of the most magnificent comets ever seen, its tail being 100 millions of miles in length and its head 127,000 miles in diameter. This wonderful phenomenon was the subject of much investigation, particularly by Olbers, the great German astronomer.