suspected that the former are represented on the moon by
the brighter and more rugged, and the latter by the smoother and more
level areas; a view, however, which Kepler more distinctly formulated in
the dictum, "Do maculas esse Maria, do lucidas esse terras." Besides
making a rude lunar chart, he estimated the heights of some of the ring-
mountains by measuring the distance from the terminator of their bright
summit peaks, when they were either coming into or passing out of
sunlight; and though his method was incapable of accuracy, and his
results consequently untrustworthy, it served to demonstrate the immense
altitude of these circumvallations, and to show how greatly they exceed
any mountains on the earth if the relative dimensions of the two globes
are taken into consideration.

Before the close of the century when selenography first became possible,
Hevel of Dantzig, Scheiner, Langrenus (cosmographer to the King of
Spain), Riccioli, the Jesuit astronomer of Bologna, and Dominic Cassini,
the celebrated French astronomer, greatly extended the knowledge of the
moon's surface, and published drawings of various phases, and charts,
which, though very rude and incomplete, were a clear advance upon what
Galileo, with his inferior optical means, had been able to accomplish.
Langrenus, and after him Hevel, gave distinctive names to the various
formations, mainly derived from terrestrial physical features, for which
Riccioli subsequently substituted those of philosophers, mathematicians,
and other celebrities; and Cassini determined by actual measurement the
relative position of many of the principal objects on the disc, thus
laying the foundation of an accurate system of lunar topography; while
the labours of T. Mayer and Schroter in the last century, and of
Lohrmann, Madler, Neison (Nevill), Schmidt, and other observers in the
present, have been mainly devoted to the study of the minuter detail of
the moon and its physical characteristics.

As was manifest to the earliest telescopic observers, its visible surface
is clearly divisible into strongly contrasted areas, differing both in
colour and structural character. Somewhat less than half of what we see
of it consists of comparatively level dark tracts, some of them very many
thousands of square miles in extent, the monotony of whose dusky
superficies is often unrelieved for great distances by any prominent
object; while the remainder, everywhere manifestly brighter, is n