will give the dates of all the four
eclipses in the year 1899. It was this rule which enabled the ancient
astronomers to predict the recurrence of eclipses, at a time when the
motions of the moon were not understood nearly so well as they now are.

During a long voyage, and perhaps in critical circumstances, the moon
will often render invaluable information to the sailor. To navigate a
ship, suppose from Liverpool to China, the captain must frequently
determine the precise position which his ship then occupies. If he could
not do this, he would never find his way across the trackless ocean.
Observations of the sun give him his latitude and tell him his local
time, but the captain further requires to know the Greenwich time before
he can place his finger at a point of the chart and say, "My ship is
here." To ascertain the Greenwich time the ship carries a chronometer
which has been carefully rated before starting, and, as a precaution,
two or three chronometers are usually provided to guard against the risk
of error. An unknown error of a minute in the chronometer might perhaps
lead the vessel fifteen miles from its proper course.

[Illustration: PLATE VI.

CHART OF THE MOON'S SURFACE.]

[Illustration: Fig. 27.--Key to Chart of the Moon (Plate VI.).]

It is important to have the means of testing the chronometers during the
progress of the voyage; and it would be a great convenience if every
captain, when he wished, could actually consult some infallible standard
of Greenwich time. We want, in fact, a Greenwich clock which may be
visible over the whole globe. There is such a clock; and, like any other
clock, it has a face on which certain marks are made, and a hand which
travels round that face. The great clock at Westminster shrinks into
insignificance when compared with the mighty clock which the captain
uses for setting his chronometer. The face of this stupendous dial is
the face of the heavens. The numbers engraved on the face of a clock are
replaced by the twinkling stars; while the hand which moves over the
dial is the beautiful moon herself. When the captain desires to test
his chronometer, he measures the distance of the moon from a
neighbouring star. For example, he may see that the moon is three
degrees from the star Regulus. In the Nautical Almanac he finds the
Greenwich time at which the moon was three degrees from Regulus.
Comparing this with the indications of the chronometer, he finds the
required co