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 afford much occupation for mathematicians. It is hardly possible that it will not be the means of lending a fresh impetus to the study of the entire Jovian system. At the same time we must remember that the detection of the new body does not offer to us, so far as we are at present advised, any interesting information of the same character as that which the satellites of Mars presented. The mass of Mars was an element not very confidently known until the satellites had been discovered, and their distances and periods measured. The mass of Jupiter, however, is one of the most carefully determined elements of the solar system. It has been accurately ascertained by the movements of its satellites, especially of the fourth. There can hardly be a doubt that the value assigned to it is right to within one-thousandth part of the whole. We have, therefore, nothing further of this kind to expect from the new satellite.

It will also be remembered that one of the most astonishing features of the Martian system of moons was the extraordinarily rapid motion of the inner of the two, by which it coursed three times round the planet before the planet turned round once. This fact at the time of its announcement was unique in the whole solar system. There was never a case known before in which a secondary planet accomplished a revolution in less time than the primary accomplished a rotation. And so far as this discovery in regard to Jupiter goes, the peculiar feature of the inner moon of Mars still remains unique. Even though the period of the satellite is only about twelve hours, it is still about two hours longer than the time which Jupiter requires to spin round his axis. We ought, however, here to notice that the rotation of