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 216 physics, inasmuch as no change could take place on the planet's surface without it, and that changes do take place is undeniable. (See page 31 et seq.)

Mars has two satellites, discovered by Hall in 1877, and known as Deimos (Dread) and Phobos (Fear), named in keeping with the God of War.

Deimos, at a distance of 14,600 miles from the planet's centre, makes his circuit in 30 hours and 18 minutes; Phobos, at a distance of 5,800, in 7 hours and 39 minutes. As Mars himself rotates in 24 hours and 39 minutes, Phobos goes round the planet faster than the planet turns upon itself, and, in consequence, would appear to any observers on the planet's surface to break the otherwise universal conformity of stellar motions by rising in the west and setting in the east. Deimos, too, is just as unconventional in its way, for it remains for two days at a time about the horizon. Furthermore, with each, owing to its nearness to the planet, its distance from any place on the surface varies at different times, and with its distance varies its apparent size in a somewhat startling manner.

As for themselves, they are very minute bodies, though not so difficult to see as is commonly stated. In the clear air of Arizona, both were conspicuous objects. They appear as stars of about the 12th and 10th magnitudes respectively; Phobos being much larger, relatively to Deimos, than its hitherto accepted value would indicate. Observations at Flagstaff by both Mr. Douglass and by me agree in making its relative brilliancy such as to give it a diameter about 3.6 times that of Deimos. It is not usually so conspicuous as Deimos, in spite of its size, because of its proximity to the planet, and the consequent much greater illumination of the field upon which it is seen. Considering their most probable albedoes as somewhat less than that of our moon,