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50 to conceive any admissible manner in which such a change could have been produced. . . . Until it can be shown with probability how on the Moon a round ring-plain some miles in diameter can be squeezed into a contorted form, the difference now existing between the two ring-plains of Messier will not in general be held to establish an instance of actual change in the formation on the surface of the Moon."

Now, singularly enough, not only was every one of these astronomers undoubtedly right in his observations, but any amateur can watch these identical changes going on from night to night before his eyes at the present time. He can watch a round "ringplain" some miles in diameter as it is apparently in the process of being " squeezed " into a great variety of contorted forms. Moreover, at different lunations these forms are by no means identical. Sometimes one crater is the larger, sometimes the other. Sometimes one or both are triangular, sometimes elliptical. When elliptical, sometimes they are parallel, sometimes nearly at right angles.

As will be shown in Chapter IX., it is probable that a varying distribution of hoarfrost, instead of a definite distribution under definite conditions, is the cause of many of the changes observed. Some of these changes are shown in the drawings in Plate D. Figures 2, 3 and 4 were all taken at about the same time in the lunar day, on different lunations, but not only their shapes but even the dark markings within them all are different.

We will now discuss what some astronomers have considered the most enigmatical feature upon the Moon's surface—the great systems of bright streaks which surround certain craters, notably Tycho, and radiate from them in all directions, in some cases for hundreds of miles (see Frontispiece). Various theories have been proposed to account for them. Nasmyth supposed them due to cracks in the surface filled by a flow of white liquid material from beneath. Neison thought them due to some process of weathering not fully explained. Würdemann suggested that they were caused by the splash of a meteorite, much like the splash that an egg might make if projected with sufficient force against a brick wall.

What has hitherto been considered one of their strangest features is that they are never visible at lunar sunrise or sunset, but require that the Sun shall have an altitude of at least five or ten degrees in order to render them visible. This peculiarity we have already explained as being due simply to the fact that the snow which forms them lies