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Age, of a planet, 206 et seq.; of Mars, 207.

Air (See Atmosphere.)

Aphelion, of Mars' orbit, 12.

Apsides, line of Martian, 22; influence on Martian seasons, 23.

Aqueous vapor, 49; boiling point of, on Mars, 59; speed of molecules of, Appendix, Note I.

Araxes, the, change in shape accounted for, 161.

Areography, 92 et seq.

Atmosphere, importance of, 31; evidence by change, 31; absence on the Moon, 32; presence on Mars, 32; measured by Mr. Douglass, 37; method of determination of, 38; quantity of, on Mars, 43; characteristics of, on Mars, 45; veiling by, 48; density of, on Mars, 50, 52, 75; boiling point of water in, 59; constitution of, 75; in relation to seeing, 139; spectroscopic observations on Martian, Appendix, Note III.

Ausonia, strait between it and Hellas, 115.

Autumn, length of Martian, 24.

Axis, tilt of Martian, 22; influence on Martian seasons, 23.

Band, surrounding polar cap, 77; composition of, 80.

Beer and Mädler, on north polar cap, 78.

Boreas, the, 62.

Bright spots, on disk, 60, 61; variability of, 62.

Brilliancy, relative, of Mars, 12.

Brontes, the, 133, 164.

Calendar, the Martian, 77

Canals, 121, 128, 129 et seq.; straightness of, 131; breadth of, 132; length of, 133, 134; history of, 136; doubling of, 137; duplication of, 140, 187 et seq.; catalogue of, 145; number of, 147; not natural features, 151, 153; changes in, 155, 157, 161, 162; constitution of, 164, 202; in dark regions, 97, 148, 171 et seq.; at times invisible, 155; list of, in dark regions, 173.

Carbonic acid gas, theory of, as related to polar cap, 80; characteristics of, 81; objection to theory of, 83; critical velocity of molecules of, Appendix Note III.

Carbonic dioxide. (See Carbonic acid gas)

Cassini, rotation of Mars, 21.

Causeways, 118.

Ceraunius, relative visibility of, 183.

Changes, on disk, 33, 34; in tint of land areas, 104, 110 et seq., 118; in canals, 155, 157, 161, 162; seasonal and secular, 161; in spots, 182.

Cimmerian Sea. (See Mare Cimmerium.)