lite is 2,000 times greater than that of the sun or moon as seen from the earth.
Periods of the Satellites.—Prof. Newcomb gives 30h and 14m as the period of the outer satellite, and 7h and 38m as that of the inner. Both move, like our moon, from west to east. The period of the inner is less, while that of the outer is greater than a Martial day. It is obvious, therefore, that, as seen from the surface of the planet, the apparent motion of the satellites will be in opposite directions, the inner rising in the west and setting in the east; the outer rising in the east and setting in the west—so that the phenomenon of two moons meeting in mid-heaven will be to the Martialists no unusual occurrence.
The Mass of Mars.—Before the discovery of these satellites the determination of the mass of Mars was a problem of great difficulty, the body being too small to have much effect in disturbing the motions of other planets. The value assigned by Burckhardt was 12680337 and that of the sun being unity. The difficulty of the problem is now happily removed, and Newcomb has found from the elements of the exterior satellite a value 13090000; mass less than Burckhardt's in the ratio of six to seven.
The Bearing of the Discovery on the Nebular Hypothesis.—The inner satellite of Mars completes three orbital revolutions in less than a Martial day. This anomalous fact in the planetary system would seem, at first view, to be utterly inconsistent with the nebular hypothesis. The question is one of more than ordinary interest, but its discussion may well be deferred until we shall have obtained more exact information in regard to the Martial system.
| HUXLEY'S AMERICAN LECTURES. |
By Professor E. RAY LANKESTER.
THE five addresses which have been recently brought out by Prof. Huxley in the form of a small volume were delivered under very varied conditions, and deal with subjects widely separate from one another. Three of them form a series of lectures on the doctrine of Evolution, and were delivered by the author at New York in September, last year. These were the only popular scientific lectures which Prof Huxley would allow himself to undertake during his summer holiday devoted to a tour in the United States. The "Address on University Education" was delivered at the formal opening of the Johns Hopkins University, at Baltimore, during the same visit. The concluding lecture of the present volume was delivered in connection
