nto coincidence,
and then separated until the apparent length of the shadow of the
mountain on the moon is equal to the distance between the lines: we then
know the number of revolutions of the micrometer screw which is
equivalent to the length of the shadow. The number of miles on the moon
which correspond to one revolution of the screw has been previously
ascertained by other observations, and hence the length of the shadow
can be determined. The elevation of the sun, as it would have appeared
to an observer at this point of the moon, at the moment when the
measures were being made, is also obtainable, and hence the actual
elevation of the mountain can be calculated. By measurements of this
kind the altitudes of other lunar objects, such, for example, as the
height of the rampart surrounding a circular-walled plane, can be
determined.

The beauty and interest of the moon as a telescopic object induces us to
give to the student a somewhat detailed account of the more remarkable
features which it presents. Most of the objects we are to describe can
be effectively exhibited with very moderate telescopic power. It is,
however, to be remembered that all of them cannot be well seen at one
time. The region most distinctly shown is the boundary between light and
darkness. The student will, therefore, select for observation such
objects as may happen to lie near that boundary at the time when he is
observing.

1. _Posidonius._--The diameter of this large crater is nearly 60 miles.
Although its surrounding wall is comparatively slender, it is so
distinctly marked as to make the object very conspicuous. As so
frequently happens in lunar volcanoes, the bottom of the crater is below
the level of the surrounding plain, in the present instance to the
extent of nearly 2,500 feet.

2. _Linne._--This small crater lies in the Mare Serenitatis. About sixty
years ago it was described as being about 6-1/2 miles in diameter, and
seems to have been sufficiently conspicuous. In 1866 Schmidt, of Athens,
announced that the crater had disappeared. Since then an exceedingly
small shallow depression has been visible, but the whole object is now
very inconsiderable. This seems to be the most clearly attested case of
change in a lunar object. Apparently the walls of the crater have
tumbled into the interior and partly filled it up, but many astronomers
doubt that a change has really taken place, as Schroeter, a Hanoverian
observer at the end of