THE PLANET MAES.

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makes them of all the more use in helping us to weigh theplanet. The methods employed before their discover)' aremuch too complicated for us to explain, and after mostlaborious investigations still involved considerable uncertaintyas to the true value. As soon, however, as the rotation-period of either of the satellites was known, it only needed avery brief calculation in order to obtain a value for the weightof Mars. Professor Asaph Hall estimates it to be equal to a_L__th part, and Professor Pritchett to a 307 ; it40 th part, of thatof the Sun. We may feel sure that either of these results isvery nearly accurate.

Our space forbids that we should attempt to discuss theastronomy of the heavens as it would be seen from Mai'S, orfrom its satellites. We cannot, however, resist the temptationto quote from The Observatoiy of December 1878 a mostinteresting description (which it was our good fortune to hearMr. Marth narrate at the preceding November meeting of theBoyal Astronomical Society) of certain phenomena which mayfrom time to time be seen by any possible inhabitants of theplanet.

“ On November 12th, 1879, shortly before two o’clock,Greenwich mean time, a small black body would make itsappearance on the south-following side of the disc of the Sun ;in six minutes it would have fully entered upon the disc, andwould proceed slowly from left to right. About a quarter pastfour o’clock, another and bigger black body would encroach uponthe disc, and would occupy twenty-one minutes before it had fullyentered upon it. The two bodies would be the Moon and theEarth, and they would he visible from all parts of Mars wherethe Sun was above the horizon. But observers placed upon acertain zone, or track, would have the opportunity of seeing athird and apparently much bigger body cross the Sun’s disc.It would come from the right-hand side in a direction at a con-siderable slant to the south, and Martial observers would haveto look sharp to observe all its contacts with the disc, since thetime for doing so would be limited to some twenty or thirtyseconds. The third body would be Phobos, the inner satellite.Martial astronomers would, however, be much more interested