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 Tempe, 60.

Terminator, observations on, 48, 63, 64; irregularities on, 63; of Moon, 169; of Mars, 170.

Tisserand, on polar flattening of Mars, 28.

Titan, the, 102, 134, 164.

Tithonius Lacus, 99, 181.

Trivium Charontis, 103, 133, 181; shape of, 179.

Twilight, on Mars, measured by Mr. Douglass, 26.

Twilight arc, of Mars, 42.

Typhon, the, double, 195.

Uranus, relative orbit and size of, 5; ellipticity of disk of, 25; evidence of atmosphere of, 57; critical velocity of, Appendix, Note I.

Variations, of surface of Mars. (See Changes.)

Vegetation, on Mars, 122, 164; visibility of, 130, 198.

Velocity, critical, of planets, Appendix, Note I.

Venus, cloud-covered, 2; ellipticity of disk of, 25; evidence of atmosphere of, 57; cusps of, 57; critical velocity of, Appendix, Note I.

Water, 79, 83, 92; necessary to life, 76, 127; theoretic reflection from, 109; amount of, on Mars, 122; direction of, when flowing from pole, 125; as fresh, 175.

Weather, on Mars, 58.

Winter, length of, on Mars, 24.

Worlds, other than our own, 3.