This tidal action was continued until the fluid became quite viscous, solidifying into little hills and ridges inside the crater, and later, as the hardening surface was dragged out of shape by the pumping of the liquid below it, little cracks were formed around the edges and across the bottom of the crater, like the rills seen in similar situations upon the Moon.
If now we raise the piston high enough and wait for air to get underneath it, we may force this down into the melted paraffine. The result is an explosion, in which the paraffine may shoot up several feet into the air. If care is taken, however, the jets may be confined to a height of a few inches. A cone is soon formed (Figure 6), and the liquid paraffine trickles down the slopes in miniature lava streams. As the cooling process goes on the paraffine comes out in bubbles, like soapsuds, which break and rapidly build up the cone. If the process is continued further, partially solid lumps of paraffine are projected into the air, falling down upon the outer slopes of the cone. The crater now gradually narrows, and if care is not taken will soon become clogged. With care many well-known volcanic phenomena may be repeated, such, for instance, as the shifting of the crater to one side and the formation of a succession of crater rings and semicircles; also the bursting out of new craters near the base of the original cone. Indeed, the investigator is likely to perform this experiment involuntarily if he permits the main vent to get partially clogged and applies too much heat below. The introduction of air seems to transport us at once from lunar to terrestrial scenes, although in the case of the Earth the tides of course have nothing to do with the matter, their place being taken by irregularly recurring explosions of steam.
Applying now the results of our experiments to the case of the Moon, we may conceive that the order of formation was somewhat as follows. We will start with the Moon in the form of a liquid or viscous sphere, revolving about the Earth and not far from it. Under these circumstances the tides would be of enormous power and quite unlike in magnitude anything at present existing upon the Earth. Those constituents of the Moon having the least specific gravity would float upon the surface and soon solidify, forming a thin crust. Whether this occurred before or after the solidification of the central core through pressure is of no consequence. As it solidified the crust would contract, forming apertures, which would soon be enlarged into circular holes or craters by the hot liquid interior. Sometimes the craters would form along a crack. We have an illustration of this in the case of the great rill of Hyginus and the small craters
