distance of fourteen thousand six hundred miles. As it takes but
little longer to complete a revolution than Mars does to rotate
on its axis, it remains in the Martial sky one hundred and
thirty-two hours between rising and setting, passing through all
the phases from new moon to full and back again four times; that
is, it swings four times around Mars before going below the
horizon. It is one of the smallest bodies discovered with a
telescope. The inner one, Phobos, is considerably larger, having
a diameter of about twenty miles. It is but twenty-seven hundred
miles from Mars's surface, and completes its revolution in seven
hours and thirty-eight minutes, which is shorter than any other
known period, Jupiter's nearest moon being the next, with eleven
hours and fifty- nine minutes. It thus revolves in less than a
third of the time Mars takes to rotate, and must consequently
rise in the west and set in the east, as it is continually
running ahead of the surface of the planet, though the sun and
all the other stars rise and set on Mars in the same way as on
the earth."

When about fifteen thousand miles from Mars, they sighted Deimos
directly ahead, and saw that they should pass on its left--i. e.,
behind--for it was moving across them. The sun poured directly
upon it, making it appear full and showing all its features.
There were small unevennesses on the surface, apparently seventy
or a hundred feet high, which were the nearest approach to
mountains, and they ran in ridges or chains. There were also
unmistakable signs of volcanic action, the craters being large
compared with the size of the planet, but shallow. They saw no
signs of water, and the blackness of the shadows convinced them
there was no air. They secured two instantaneous photographs of