lava that was fluid, at least where the sunshine was most intense. The weakest point on the planet was therefore the one just opposite the Sun. When later collapses occurred the cracks commenced at this weak point. We see on the figure how not less than six fissures radiate from this centre. Others were formed where the crust broke off from adjacent solid portions. These latter fissures have a less rectilinear appearance than those diverging from the centre of collapse. Along these faults, reducing gases no doubt issue from the interior of the planet and give a dark tone to the surface layers, which probably consist of ferruginous dust falling from space. In the neighbourhood of the Sun, such dust ought to be more plentiful, concentrated as it were by the gravitation of the Sun. Mercury lies five times nearer the Sun than the Earth does, and twelve times nearer than Mars. There probably also exist on Mercury, as on the Moon, large mountains which are not subject to the wear of running water and blowing sand. We cannot, however, observe them from the Earth. Possibly they correspond to the widely extended spots, which several investigators as Schroeter, Vogel, and others, have noticed, formations resembling the “seas” on the Moon. Vogel believed that he had found traces of water vapour in the atmosphere of Mercury as in that of Mars, a belief in both cases undoubtedly founded on erroneous observations. [Illustration: Fig. 25. Drawing by Lowell, representing the planet Mercury with “canals.” ] [Illustration: Fig. 26. A part of the moon near its south pole. The big crater above, in the interior and on the walls of which a large number of smaller craters appear, is Clavius. A little below and to the right is Longomontanus just at the edge of the shadow; almost in the middle of the picture appears Tycho with its central cone. The moon diameter corresponds to 43.4 cm. Photo by Yerkes Observatory. ] [Illustration: Fig. 27. Mare Serenitatis (below), Mare Tranquillitatis (upper left) and vicinity. To the left of Mare Serenitatis the great crater Posidonius; 2.8 cm. from the right edge a small white spot may be seen. This is the remarkable crater Linné, said to have undergone changes. The moon diameter corresponds to 35.7 cm. Photo by Yerkes Observatory. ]