The Moon and the Tides--The Use of the Moon in Navigation--The Changes of the Moon--The Moon and the Poets--Whence the Light of the Moon?--Sizes of the Earth and the Moon--Weight of the Moon--Changes in Apparent Size--Variations in its Distance--Influence of the Earth on the Moon--The Path of the Moon--Explanation of the Moon's Phases--Lunar Eclipses--Eclipses of the Sun, how produced--Visibility of the Moon in a Total Eclipse--How Eclipses are Predicted--Uses of the Moon in finding Longitude--The Moon not connected with the Weather--Topography of the Moon--Nasmyth's Drawing of Triesnecker--Volcanoes on the Moon--Normal Lunar Crater--Plato--The Shadows of Lunar Mountains--The Micrometer--Lunar Heights--Former Activity on the Moon--Nasmyth's View of the Formation of Craters--Gravitation on the Moon--Varied Sizes of the Lunar Craters--Other Features of the Moon--Is there Life on the Moon?--Absence of Water and of Air--Dr. Stoney's Theory--Explanation of the Rugged Character of Lunar Scenery--Possibility of Life on Distant Bodies in Space. If the moon were suddenly struck out of existence, we should be immediately apprised of the fact by a wail from every seaport in the kingdom. From London and from Liverpool we should hear the same story--the rise and fall of the tide had almost ceased. The ships in dock could not get out; the ships outside could not get in; and the maritime commerce of the world would be thrown into dire confusion. The moon is the principal agent in causing the daily ebb and flow of the tide, and this is the most important work which our satellite has to do. The fleets of fishing boats around the coasts time their daily movements by the tide, and are largely indebted to the moon for bringing them in and out of harbour. Experienced sailors assure us that the tides are of the utmost service to navigation. The question as to how the moon causes the tides is postponed to a future chapter, in which we shall also sketch the marvellous part which the tides seem to have played in the early history of our earth.