Page:The Moon (Pickering).djvu/48

28 like paraffine This substance melts at so low a temperature that it can be readily handled in the viscous form, while at the same time it becomes hard and firm at ordinary temperatures. Like slag and the materials composing the crust of the Earth, it contracts on solidifying, the change in volume in each case being quite large.

The paraffine was melted in an enameled-ware pan, measuring three and a half inches deep by eight in diameter, over a small spirit lamp. By employing a small source of heat the paraffine was melted locally above the flame and soon formed a little hole in the surface crust measuring about one-quarter of an inch in diameter. That portion of the liquid in contact with the bottom of the pan was at a much higher temperature than that above it, and was forced upward by the heat, rapidly enlarging the hole formed in the upper crust. The hole retained its circular or elliptical form, and continued to enlarge as long as the hot liquid was brought in contact with it. As soon as it had reached a convenient size the lamp was extinguished and the cooling process begun. As the lower regions of the paraffine cooled they contracted, and the liquid surface dropped, leaving a smoothly cut elliptical pit (Plate A, Figure 3). The sides were at first quite shelving, but by reheating the fluid once or twice they became steeper, and even overhung in some places. Probably a slower cooling at the surface and a more rapid contraction of the fluid, obtained by using a larger reservoir, would accomplish the same result. If the contraction is allowed to proceed too far, however, the floor of the crater pit becomes concave and may even be broken through by the pressure of the atmosphere.

In order to imitate the former powerful influence of the tides upon the liquids contained within and upon the surface of the Moon, a brass tube one inch in diameter and twelve inches long was inserted in the paraffine when it was first melted. The tube was fitted with a wooden piston packed loosely with cotton flannel. By working this piston up and down the melted paraffine could be made alternately to rise and fall inside the craters formed by it, and the cooling process could be hastened when desired by blowing upon the liquid surface. Craters (Figures 4 and 5) were formed in the same manner as the first one, excepting that after extinguishing the lamp the tidal action was brought into play, alternately pumping the liquid up to the rim of the crater, where it partially solidified, leaving a little ring of solid paraffine, and then drawing it down again into the interior, where it soon partly remelted, preparatory to a renewed elevation.