The climate of Mars, according to Schiaparelli, “must resemble that of a clear day upon a high mountain. By day a very strong solar radiation hardly at all mitigated by mist or vapor; by night a copious radiation from the soil toward celestial space, and hence a very marked refrigeration; consequently, a climate of extremes, and great changes of temperature from day to night, and from one season to another. And as on the earth, at altitudes of from 17,000 to 20,000 feet, the vapor of the atmosphere is condensed only into the solid form, producing those whitish masses of suspended crystals which we call cirrus-clouds, so in the atmosphere of Mars it would be rarely possible to find collections of cloud capable of producing rain of any consequence. The variation of temperature from one season to another would be notably increased by their long duration, and thus we can understand the great freezing and melting of the snow, renewed in turn at the poles at each complete revolution of the planet round the sun.” The German astronomer Mädler searched in 1830 for a Martian satellite, and although his telescope was of less than four inches aperture, he satisfied himself that none with a diameter of as much as twenty-three miles could be in existence. As it happened, he was right. The pair of moons detected by Professor Asaph Hall with the Washington twenty-six refractor, August 11 and 17, 1877, are unquestionably below that limit of size. Neither of them can well be more than ten miles across. Their names, “Deimos” and “Phobos,” are taken from the _Iliad_, where Fear and Panic are introduced as attendants upon the God of War. Deimos revolves in 30 hours and 18 minutes at a distance of 14,600 miles from the centre of Mars. And since the planet rotates in 24 hours, 37 minutes, the diurnal motion of the sphere from east to west is so nearly neutralized by the orbital circulation of the satellite from west to east that nearly 132 hours elapse between its rising and its setting. During the interval, it changes four times from new to full, and _vice versâ_. Phobos is more effective in illumination, both because it is larger and because it is less distant. At the Martian equator, its brightness is equal to 1/60th that of our moon, but beyond 69° of latitude it is permanently shut out from view by the curvature of the globe. THE PLANETOIDS.—CAMILLE FLAMMARION