ther known case where the satellite revolves around the primary more
quickly than the primary rotates on its axis. The anomalous movement of
the satellite of Mars has, however, been accounted for. In a subsequent
chapter we shall again allude to this, as it is connected with an
important department of modern astronomy.

The satellites are so small that we are unable to measure their
diameters directly, but from observations of their brightness it is
evident that their diameters cannot exceed twenty or thirty miles, and
may be even smaller. Owing to their rapid motion the two satellites must
present some remarkable peculiarities to an observer on Mars. Phobos
rises in the west, passes across the heavens, and sets in the east after
about five and a half hours, while Deimos rises in the east and remains
more than two days above the horizon.

As the satellites revolve in paths vertically above the equator of their
primary, the one less than 4,000 miles and the other only some 14,500
miles above the surface, it follows that they can never be visible from
the poles of Mars; indeed, to see Phobos, the observer's planetary
latitude must not be above 68-3/4 deg.. If it were so, the satellite would
be hidden by the body of Mars, just as we, in the British Islands, would
be unable to see an object revolving round the earth a few hundred miles
above the equator.

Before passing from the attractive subject of the satellites, we may
just mention two points of a literary character. Mr. Hall consulted his
classical friends as to the designation to be conferred on the two
satellites. Homer was referred to, and a passage in the "Iliad"
suggested the names of Deimos and Phobos. These personages were the
attendants of Mars, and the lines in which they occur have been thus
construed by my friend Professor Tyrrell:--

"Mars spake, and called Dismay and Rout
To yoke his steeds, and he did on his harness sheen."

A curious circumstance with respect to the satellites of Mars will be
familiar to those who are acquainted with "Gulliver's Travels." The
astronomers on board the flying Island of Laputa had, according to
Gulliver, keen vision and good telescopes. The traveller says that they
had found two satellites to Mars, one of which revolved around him in
ten hours, and the other in twenty-one and a half. The author has thus
not only made a correct guess about the number of the satellites, but he
actually stated the periodic time with c