small ones. So, in the case of the lunar craters, you find the large ones studded over with small ones, and these, again, have craters of a third order of smallness. If you throw successive handfuls of pebbles into a pond, you will see, at the same time, circles interlacing with one another, and smaller ones diversifying, in every imaginable way, the larger ones. Precisely such a spectacle is presented by the structure of the moon. Theories of volcanic action have been proposed to explain all this; but we cannot linger longer on the summit of the central peak, and as there is no other mode of egress, we shall take an imaginary flight over the encircling wall, and again alight on the vast savannah from which we ascended.
We studiously avoided making any remarks upon the nature of this surface when we previously passed over it, as we could really offer no plausible account of it. But so rapid is the advance of science, even in the department of astronomy, that, in the interval between the ascent and descent we have learned that Father Secchi, of the observatory at Rome, has made a curious discovery on the subject. Surfaces reveal their nature, to a certain extent, by reflecting certain colours in perference to others, but they also give a clue to their character by the various ways in which they polarise light. Now, Secchi has found equally polarised light in the wdiole smooth, dark plains of the moon, whatever be the inclination of the incident and reflected rays. The only substance we are
