onomy are of two kinds: One relates to
geography; the other to times, seasons, and chronology. Every navigator
who sails long out of sight of land must be something of an astronomer.
His compass tells him where are east, west, north, and south, but it
gives him no information as to where on the wide ocean he may be, or
whither the currents may be carrying him. Even with the swiftest modern
steamers it is not safe to trust to the compass in crossing the
Atlantic. A number of years ago the steamer City of Washington set out
on her usual voyage from Liverpool to New York. By rare bad luck the
weather was stormy or cloudy during her whole passage, so that the
captain could not get a sight on the sun, and therefore had to trust to
his compass and his log-line, the former telling him in what direction
he had steamed, and the latter how fast he was going each hour. The
result was that the ship ran ashore on the coast of Nova Scotia, when
the captain thought he was approaching Nantucket.

Not only the navigator but the surveyor in the western wilds must
depend on astronomical observations to learn his exact position on the
earth's surface, or the latitude and longitude of the camp which he
occupies. He is able to do this because the earth is round, and the
direction of the plumb-line not exactly the same at any two places. Let
us suppose that the earth stood still, so as not to revolve on its axis
at all. Then we should always see the stars at rest and the star which
was in the zenith of any place, say a farm-house in New York, at any
time, would be there every night and every hour of the year. Now the
zenith is simply the point from which the plumb-line seems to drop. Lie
on the ground; hang a plummet above your head, sight on the line with
one eye, and the direction of the sight will be the zenith of your
place. Suppose the earth was still, and a certain star was at your
zenith. Then if you went to another place a mile away, the direction of
the plumb-line would be slightly different. The change would, indeed,
be very small, so small that you could not detect it by sighting with
the plumb-line. But astronomers and surveyors have vastly more accurate
instruments than the plumb-line and the eye, instruments by which a
deviation that the unaided eye could not detect can be seen and
measured. Instead of the plumb-line they use a spirit-level or a basin
of quicksilver. The surface of quicksilver is exactly level and so at
right an