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30 distributed along its length. Hyginus is probably a later formation, however, as, if the crust had been thin, the craters would have been larger.

For the convenience of the reader, in the description of the various craters and other formations, a reference is given in each case to the plate at the end of the book on which the crater is shown to the best advantage. The position of the crater on this plate is indicated by two numbers placed in brackets. The first number gives the distance of the crater in inches and tenths from the left-hand edge of the plate and the second its distance from the bottom. Thus, Hyginus may be found on Plate 7E [2.1, 7.0].

It does not follow, however, that the statement made in the text can in every case be verified on the photograph. The best photographs ever made give by no means as satisfactory a view of the Moon as can be obtained by a telescope of even five inches in diameter, under suitable atmospheric conditions, and the more delicate features upon the Moon cannot be found on any photograph.

When the process of solidification first began on the Moon numerous comparatively small holes would form one after another. These holes would continue to enlarge, retaining their circular form, as the hot liquid was forced through them, until the action was stopped by a sufficiently thick crust forming upon the liquid surface. In the meantime, the tremendous tides engendered by our Earth, coursing through the imprisoned fluid interior, would fracture the thin and brittle crust in fresh places, where the same process would be repeated. When the crust was thin the enlargement of the crater would proceed rapidly, and the aperture might attain considerable dimensions before the restraining crust was formed, but as the original crust thickened and the passage connecting the aperture with the liquid interior lengthened we should find that the craters formed would be smaller but more numerous. We should thus expect, in general, that the older the crater the larger it would be, and that the smaller craters would impinge upon the larger ones, and not vice versa. An examination of the limar surface, as we have just seen, shows this to be the case. The older and larger craters, like Clavius, 10A [2.1, 7.4], Albategnius, 8A [2.5, 2.8], and many others near the south pole, are pitted and sometimes almost concealed by numerous smaller and later craters, while craters of more moderate size, like Tycho, 10A [2.2, 6.2], Copernicus11A [2.5, 6.9], and others still smaller, are comparatively free from such intrusions.

It can be shown that the maximum surface tension exerted by the Earth upon the Moon is produced upon the great circle forming the limb, and tends to separate the two