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132 and that we might be able to detect the effect. Again the extremes of temperature to which the surface is subjected may result in some cracking, tho it is doubtful whether this could proceed on a scale large enough to become visible. Physical changes, particularly in connection with the small crater Linné, have been reported by some expert observers; but others say that "no eye has ever seen a physical change in the plastic features of the Moon's surface."

Definite and very positive statements are made by some competent observers that slight color changes take place in the course of each month in the neighborhood of certain of the craters, and some have ascribed these to the issuing of vapors from the cones. Because there is no atmosphere, these vapors would be immediately deposited as snow or hoar frost in the lunar night and evaporated during the lunar day, and would thus explain the observed changes in appearance. More data are necessary before any certain conclusion can be reached.

This is true even of the nature of the general surface of the Moon. Most authorities agree that the surface is a dry barren desert of rock and debris covered with a deposit of meteoric dust such as exists in our own atmosphere and is found on the snows of the polar regions and even on the bottom of the ocean; but there are others who think it is covered with a thick deposit of ice! The Moon may be a dead world, but it clear that it will long continue to be an interesting body to study.


 * February, 1917.