out 64 miles in diameter, with an internal depth of
14,000 to 18,000 feet, and a beautiful central group of mountains,
one-third of that height, on its floor. This proves that the last
eruptive efforts in this part of the moon fully equalled in intensity
those that had preceded them. Although Theophilus is on the whole the
deepest crater we can see in the moon, it has received little or no
deformation by secondary eruptions."

But perhaps the most remarkable object on the whole hemisphere of the
moon is "the majestic Tycho," which rises from the surface near the
south pole, and at a distance of about 1/6th of the diameter of the
sphere from its margin. Its depth is stated by Ball to be 17,000 feet,
and its diameter 50 miles. But its special distinction amongst the other
volcanic craters lies in the streaks of light which radiate from it in
all directions for hundreds and even thousands of miles, stretching with
superb indifference across vast plains, into the deepest craters, and
over the highest opposing ridges. When the sun rises on Tycho these
streaks are invisible, but as soon as it has reached a height of 25 deg. to
30 deg. above the horizon, the rays emerge from their obscurity, and
gradually increase in brightness until full moon, when they become the
most conspicuous objects on her surface. As yet no satisfactory
explanation has been given of the origin of these illuminated rays,[10]
but I may be permitted to add that their form and mode of occurrence are
eminently suggestive of gaseous exhalations from the volcano illumined
by the sun's rays; and owing to the absence of an atmosphere, spreading
themselves out in all directions and becoming more and more attenuated
until they cease to be visible.

The above account will probably suffice to give the reader a general
idea of the features and inferential structure of the moon's surface.
That she was once a molten mass is inferred from her globular form; but,
according to G. F. Chambers, the most delicate measurements indicate no
compression at the poles.[11] That her surface has cooled and become
rigid is also a necessary inference; though Sir J. Herschel considered
that the surface still retains a temperature _possibly_ exceeding that
of boiling water.[12] However this may be, it is pretty certain that
whatever changes may occur upon her surface are not due to present
volcanic action, all evidence of such action being admittedly absent.
If, when the earth and moo