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Flammarion finally submits the speculation that the undiscovered planet must, if it be related to the comets of the 5th group, revolve at somewhere about twice the distance of Neptune, say, in a period of 300 years.

Forbes's speculations do not in any way fit in with Flammarion's. Forbes gives his planet a mean distance of 100 radii of the Earth's orbit—more than double the distance assigned by Flammarion. And his group of comets is not constituted as Flammarion's is. They are 8 in number, viz.: 1556, 1840 (iv.), 1855 (i.), 1855 (ii.), 1861 (i.), 1843 (i.), 1880 (i.), and 1882 (ii.), but Forbes treats the last 3 as fragments resurrected of the Comet of 1556, which seems to have disappeared: at any rate it did not return in 1848 as expected.

To complete the information respecting families of comets, it may be stated that the reason why the smaller planets near the Sun, Mercury, Venus, the Earth, and Mars have no comets under their control would seem to be that their masses (i.e. their powers of attraction) are so much less than the masses of the much larger distant planets; and, moreover, because comets coming up to the Sun are moving through our neighbourhood at speeds much greater than they are endued with when passing in the vicinity of the more distant planets, and can therefore more easily run away out of reach of enemies (e.g. planets).

When a comet is discovered the first questions asked about it by the ordinary searcher after knowledge is, "When and where can we see it?" "How long will it last?" and "Has it got a tail?"—whilst the professional astronomer wants to know, "What are its elements?" The answers to be given to the first two questions always depend upon the answer which has been given to the last question. To the majority