To their practical, common sense vision _Mare Nubium_, the Cloudy Sea, was an immense depression of the surface, sprinkled here and there with a few circular mountains. Covering a great portion of that part of the southern hemisphere which lies east of the centre, it occupied a space of about 270 thousand square miles, its central point lying in 15 deg. south latitude and 20 deg. east longitude. Northeast from this lay _Oceanus Procellarum_, the Ocean of Tempests, the most extensive of all the plains on the lunar disc, embracing a surface of about half a million of square miles, its centre being in 10 deg. north and 45 deg. east. From its bosom those wonderful mountains _Kepler_ and _Aristarchus_ lifted their vast ramparts glittering with innumerable streaks radiating in all directions. To the north, in the direction of _Mare Frigoris_, extends _Mare Imbrium_, the Sea of Rains, its central point in 35 deg. north and 20 deg. east. It is somewhat circular in shape, and it covers a space of about 300 thousand square miles. South of _Oceanus Procellarum_ and separated from _Mare Nubium_ by a goodly number of ring mountains, lies the little basin of _Mare Humorum_, the Sea of Humors, containing only about 66 thousand square miles, its central point having a latitude of 25 deg. south and a longitude of 40 deg. east. On the shores of these great seas three "Gulfs" are easily found: _Sinus Aestuum_, the Gulf of the Tides, northeast of the centre; _Sinus Iridium_, the Gulf of the Rainbows, northeast of the _Mare Imbrium_; and _Sinus Roris_, the Dewy Gulf, a little further northeast. All seem to be small plains enclosed between chains of lofty mountains.