egular outline, 24 miles in diameter,
situated in the midst of the Taurus highlands. It has a very large
central mountain, a crater on the N. side of the floor, and terraced
inner slopes. Some distance on the N. is another ring, nearly as large,
with a crater on its S. rim, and between this and Posidonius is another
with a wide gap on the S. and a crater on its N. border. One of the most
remarkable crater-rills on the moon runs from the E. side of Romer
through this latter ring, and then northwards on to the plain W. of
Posidonius. Under suitable conditions, it can be seen as such in a 4 inch
achromatic. It is easily traceable as a rill in a photograph of the N.
polar region of the moon taken by MM. Henry at the Paris Observatory, and
recently published in _Knowledge_.

LE MONNIER.--A great inflection or bay on the W. border of the Mare
Serenitatis S. of Posidonius. Like many other similar formations on the
edges of the Maria, it appears at one time or other to have had a
continuous rampart, which on the side facing the "sea" has been
destroyed. In this, as in most of the other cases, relics of the ruin are
traceable under oblique light. A fine crescent-shaped mountain, 3000 feet
high, stands near the S. side of the gap, and probably represents a
portion of a once lofty wall. It will repay the observer to watch the
progress of sunrise on the whole of the W. coast-line of the Mare up to
Mount Argaeus.

POSIDONIUS.--This magnificent ring-plain is justly regarded as one of the
finest telescopic objects in the first quadrant. Its narrow bright wall
with its serrated shadow, the conspicuous crater, the clefts and ridges
and other details on the floor, together with the beautiful group of
objects on the neighbouring plain, and the great Serpentine ridge on the
E., never fail to excite the interest of the observer. The
circumvallation, which is far from being perfectly regular, is about 62
miles in diameter, and, considering its size, is not remarkable for its
altitude, as it nowhere exceeds 6000 feet above the interior, which is
depressed about 2000 feet below the surrounding plain. Its continuity,
especially on the E., is interrupted by gaps. On the N., the wall is
notably deformed. It is broader and more regular on the W., where it
includes a large longitudinal depression, and on the N.W. section stand
two bright little ring-plains. On the floor, which shines with a
glittering lustre, are the well-marked remains of a secon