The Moon | Page 4

undoubted existence of the relics of an earlier lunar
world beneath their smooth superficies.
MARIA.--Leaving, however, these considerations for a more particular
description of the Maria, it is clearly impossible, in referring to their
level relatively to the higher and brighter land surface of the moon, to
appeal to any hypsometrical standard. All that is known in this respect
is, that they are invariably lower than the latter, and that some sink to a
greater depth than others, or, in other words, that they do not all form a

part of the same sphere. Though they are more or less of a greyish-slaty
hue--some of them approximating very closely to that of the pigment
known as "Payne's grey"--the tone, of course, depends upon the angle
at which the solar rays impinge on that particular portion of the surface
under observation. Speaking generally, they are, as would follow from
optical considerations, conspicuously darker when viewed near the
terminator, or when the sun is either rising or setting upon them, than
under a more vertical angle of illumination. But even when it is
possible to compare their colour by eye-estimation under similar solar
altitudes, it is found that not only are some of the Maria, as a whole,
notably darker than others, but nearly all of them exhibit local
inequalities of hue, which, under good atmospheric and instrumental
conditions, are especially remarkable. Under such circumstances I have
frequently seen the surface, in many places covered with minute
glittering points of light, shining with a silvery lustre, intermingled with
darker spots and a network of streaks far too delicate and ethereal to
represent in a drawing. In addition to these contrasts and differences in
the sombre tone of these extended plains, many observers have
remarked traces of a yellow or green tint on the surface of some of
them. For example, the Mare Imbrium and the Mare Frigoris appear
under certain conditions to be of a dirty yellow-green hue, the central
parts of the Mare Humorum dusky green, and part of the Mare
Serenitatis and the Mare Crisium light green, while the Palus Somnii
has been noted a golden-brown yellow. To these may be added the
district round Taruntius in the Mare Foecunditatis, and portions of
other regions referred to in the catalogue, where I have remarked a very
decided sepia colour under a low sun. It has been attempted to account
for these phenomena by supposing the existence of some kind of
vegetation; but as this involves the presence of an atmosphere, the idea
hardly finds favour at the present time, though perhaps the possibility
of plant growth in the low-lying districts, where a gaseous medium may
prevail, is not altogether so chimerical a notion as to be unworthy of
consideration. Nasmyth and others suggest that these tints may be due
to broad expanses of coloured volcanic material, an hypothesis which,
if we believe the Maria to be overspread with such matter, and knowing
how it varies in colour in terrestrial volcanic regions, is more probable
than the first. Anyway, whether we consider these appearances to be

objective, or, after all, only due to purely physiological causes, they
undoubtedly merit closer study and investigation than they have
hitherto received.
There are twenty-three of these dusky areas which have received
distinctive names; seventeen of them are wholly, or in great part,
confined to the northern, and to the south-eastern quarter of the
southern hemisphere--the south-western quadrant being to a great
extent devoid of them. By far the largest is the vast Oceanus
Procellarum, extending from a high northern latitude to beyond latitude
10 deg. in the south-eastern quadrant, and, according to Schmidt, with
its bays and inflections, occupying an area of nearly two million square
miles, or more than that of all the remaining Maria put together. Next
in order of size come the Mare Nubium, of about one-fifth the
superficies, covering a large portion of the south-eastern quadrant, and
extending considerably north of the equator, and the Mare Imbrium,
wholly confined to the northeastern quadrant, and including an area of
about 340,000 square miles. These are by far the largest lunar "seas."
The Mare Foecunditatis, in the western hemisphere, the greater part of
it lying in the south- western quadrant, is scarcely half so big as the
Mare Imbrium; while the Maria Serenitatis and Tranquilitatis, about
equal in area (the former situated wholly north of the equator, and the
latter only partially extending south of it), are still smaller. The arctic
Mare Frigoris, some 100,000 square miles in extent, is the only
remaining large sea,--the rest, such as the Mare Vaporum, the Sinus
Medii, the Mare Crisium, the Mare Humorum, and the Mare
Humboldtianum, are of comparatively small dimensions, the Mare
Crisium not greatly exceeding 70,000 square miles, the Mare
Humorum (about the size