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 Clerk-Maxwell, determination of molecular speed, 54.

Climate, of Mars, 49.

Cloudlessness, of Martian skies, 45.

Clouds, 45, 62; how formed, 52; as seen on terminator, 64, 65; kinds, 68; curious example of, seen by Mr. Douglass, 70 et seq.; height of same, 73; movement of same, 74; no trace of veiling of surface by, 157.

Color, of Mars, 121; of land areas, 177.

Comet-tail peninsulas, 103.

Cooling, of the Earth, 30; of Mars, 30.

Cracks, theory of the canals as, 152.

Critical velocity, defined, 55; of the Earth, 56, Appendix, Note I.; of Mercury, 58, Appendix, Note I.; of Mars, 58, Appendix, Note I.; of Venus, of the Moon, of Jupiter, of Saturn, of Uranus, of Neptune, of the Sun, Appendix, Note I.

Cyane Fons, relative visibility of, 183.

Dardanus, the, double, 196.

Dark areas, depressions over, 69.

Day, of Mars, 22.

Deimos, orbit and size, Appendix, Note IV.

Density, of Mars, 19.

Denudation, far advanced on Mars, 171, 207.

Depressions, on terminator, 65; on terminator, over dark areas, 69.

Deserts, of Mars, 60, 108.

Deucalionis Regio, neck between it and Fastigium Aryn, 118.

Development, seasonal, in seas, 122; of canals, 154 et seq.

Diameter, of Mars, equatorial, 16, 42; polar, 42; Appendix, Note II.

Distance, relative, of Mars from Earth, 4, 12.

Double canals, on map, 106 ; account of, 188 et seq.; appearance of, 189; width between, 190; manner of doubling, 190, 192.

Douglass, A. E., measurement of diameters, 41; cloud observations, 70; rift in polar cap, 87; observations on the Nectar, 194.

Duplication, of canals. (See Gemination.)

Earth, ellipticity of the, 25; critcal velocity of the, 56, Appendix, Note I.

Eccentricity, of orbit of Mars, 24; of the Earth, 24.

Elysium, 60, 61.

Equator, inclination of Martian, to orbit, 77.

Equinoxes, 77; vernal equinox of Martian southern hemisphere, 114; autumnal equinox of Martian southern hemisphere, 114.

Eridania, 60.

Eumenides, length of the, 133; oases detected on the, 181.

Eunostos, the, 62.

Euphrates, the, shown double, 106; doubling of, 194.

Faraday, experiments on carbonic acid gas, 81.

Fastigium Aryn, the, origin of Martian longitudes, 95 ; neck between it and Deucalionis Regio, 118.

Flammarion, Camille, "La Planète Mars," 111.

Fons Juventae, the, 96, 180, 193.

Frost, possible effect of, on Mars, 59, 62.