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22 not be moving very rapidly with regard to the Moon, would in general strike nearly perpendicular to the surface. The two prominent objections to the meteoric theory would therefore be removed. He caused drops of water to fall on a surface of thin mud, producing craters with walls and also in some cases with central summits. Dr. W. S. Bigelow, of Boston, carried on similar experiments, in which he fired pistol bullets into hardening plaster of Paris. R. S. Tozer has recently projected clay balls against a clay surface, thereby producing craters very similar in some respects to those found upon the Moon. A photograph showing some of his results will be found on Plate A, Figure 1.

There are two objections to this theory, however, which cannot be readily surmounted. If the Earth were originally surrounded by a swarm of bodies which gradually coalesced by collision to form our present Moon, the earlier collisions would be those of small bodies. As time went on, larger and larger bodies would drop into the Moon, and we should accordingly expect, when two craters happened to overlap, that in many cases the larger crater, being of more recent origin, would partially obliterate the smaller one. If, now, we examine carefully the plates illustrating this volume, we shall find hundreds of cases where small craters have impinged upon larger ones, the wall of the larger crater being in some cases totally destroyed by the smaller and later formation. I have so far found only two instances where a large crater has impinged upon a smaller one. One example will be found on Plate 1A, just to the right of the centre. In the other case the two craters lie between Zach and Curtius, and are well shown near the top of Plate 8E. It will be noted, moreover, that in neither of these cases is there much difference in the size of the impinging craters.

The other objection to this theory depends on the laws of viscosity of matter. We know that certain solids like moist clay will yield to pressure, and may be formed into craters. We know, also, that many solids, like iron, for instance, if uniformly heated within a rather narrow range of temperature, will also yield to pressure. Again, still other materials, like the rocks of our terrestrial crust, will yield if the pressure is great enough and is applied very slowly through a period of ages. But we know that these same rocks will break and not yield, no matter what the pressure, if it is applied suddenly.

We therefore conclude that, unless the temperature was exactly right and uniformly distributed, the lunar craters could not have been formed by collision, and if the