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