It must be conceded that the profoundest study has not mastered the whole philosophy of tides.  There are certain facts which are apparent, but for an explanation of their true theory such men as Laplace, Newton, and Airy have labored in vain.  There are plenty of other worlds still to conquer.

[Page 150] [Illustration:  Fig. 57.—­Lunar Day.]

[Page 151] THE MOON.

New moon, [Symbol]; first quarter, [Symbol]; full moon, [Symbol]; last quarter, [Symbol].

EXTREME DISTANCE FROM THE EARTH, 259,600 MILES; LEAST, 221,000 MILES; MEAN, 240,000 MILES.  DIAMETER, 2164.6 MILES [2153, LOCKYER].  REVOLUTION ABOUT THE EARTH, 29-1/2 DAYS.  AXIAL REVOLUTION, SAME TIME.

When the astronomer Herschel was observing the southern sky from the Cape of Good Hope, the most clever hoax was perpetrated that ever was palmed upon a credulous public.  Some new and wonderful instruments were carefully described as having been used by that astronomer, whereby he was enabled to bring the moon so close that he could see thereon trees, houses, animals, and men-like human beings.  He could even discern their movements, and gestures that indicated a peaceful race.  The extent of the hoax will be perceived when it is stated that no telescope that we are now able to make reveals the moon more clearly than it would appear to the naked eye if it was one hundred or one hundred and fifty miles away.  The distance at which a man can be seen by the unaided eye varies according to circumstances of position, background, light, and eye, but it is much inside of five miles.

Since, however, the moon is our nearest neighbor, a member of our own family in fact, it is a most interesting object of study.

A glance at its familiar face reveals its unequal illumination.  All ages and races have seen a man in the moon.  All lovers have sworn by its constancy, and only part of them have kept their oaths.  Every twenty-nine or thirty days we see a silver crescent in the west, and are glad if it comes over the right shoulder—­so [Page 152] much tribute does habit pay to superstition.  The next night it is thirteen degrees farther east from the sun.  We note the stars it occults, or passes by, and leaves behind as it broadens its disk, till it rises full-orbed in the east when the sun sinks in the west.  It is easy to see that the moon goes around the earth from west to east.  Afterward it rises later and smaller each night, till at length, lost from sight, it rises about the same time as the sun, and soon becomes the welcome crescent new moon again.

The same peculiarities are always evident in the visible face of the moon; hence we know that it always presents the same side to the earth.  Obviously it must make just one axial to one orbital revolution.  Hold any body before you at arm’s-length, revolve it one-quarter around you until exactly overhead.  If it has not revolved on an axis between the hands, another quarter of the surface is visible; but if