Page:The Moon (Pickering).djvu/47

 CHAPTER V

to Captain Dutton, the origin of the Hawaiian craters is due to the collapse and falling in of the mountain summits. In this view we can hardly follow him, as a localised melting of the Earth's crust would seem a more natural explanation. As illustrative of this matter, we will now describe a very simple experiment, which is or rather was formerly constantly performed in the ordinary processes of manufacture at our great iron works. It was early noticed by several different persons that when a large mass of iron slag is permitted to solidify small apertures will form in its surface resembling in some respects the craters on the Moon. One of these earlier observers, Mr. J. A. Brashear, made a special study of this subject, and to him I am indebted for the following description and for the photograph which appears in Plate A, Figure 2. The crater there represented was formed naturally without any human intervention whatever. The mass of slag on the surface of which it appeared was somewhat conical in form, measuring about four feet in diameter by one foot in depth at the centre, and weighing perhaps 800 pounds. The rim of the crater measured three and a half inches in diameter. Mr. Brashear states that it is rare for a mass of slag of this size to cool without forming some sort of a crater. The crater only appears after the whole surface has solidified. The contraction accompanying this solidification causes the liquid interior, aided somewhat by the gases formed at the time, to burst through the solid crust and gradually to build up the crater walls. The fluid then subsides, leaving in some cases terraces. The floor next solidifies, sometimes again bursting through to form minute craterlets, and sometimes cracks in the hardening surface. On breaking up the slag, large cavities are usually found under the craters, and in one instance a secondary crater similar and of the same size as the first was found directly beneath it.

Since iron slag is a somewhat difficult substance to manipulate, it occurred to the author some years ago that one might conveniently substitute for it some material 27