The incandescent sphere having attained its maximum degree of heat by the total conversion thereinto of the gravitating force it embodied, we explained how there must have ensued a dispersion of that heat by radiation into surrounding space, resulting in the cooling and consequent solidification of the outermost stratum of the lunar sphere, and subsequently in the continuation of the cooling process downwards or inwards to the centre. And here we essayed to prove that in this second stage of the cooling process, when the crust was solid and the subjacent portion of the molten sphere was about to solidify, there would come into operation a principle which appears to govern the behaviour of certain fusible substances, and which may be concisely termed the principle of pre-solidifying expansion. We adduced several examples of the manifestation of this principle, soliciting for it the careful consideration of physicists and geologists, and looking to it as furnishing the key to the mystery of volcanic action upon the moon, since, without needing recourse to aqueous or gaseous sources of eruptive power, it afforded a rationale of the ejection of the fluid and semifluid matter of the moon through the solidified crust thereof, and also of the dislocations of that crust, unattended by actual ejection of subsurface matter, of which our satellite presents a variety of examples, and which the earth also appears to have experienced at some period of its formative history. Arrived at this stage of our subject we thought it needful to introduce some pages of data and descriptive detail. Accordingly in one chapter we discussed the form, magnitude, weight, and density of the moon, and the force of gravity at its surface: and the more soundly to fix these data in the mind, we devoted a few lines to explanation of the methods whereby each has been ascertained. We then examined the question (so important to our subject) of the existence or non-existence of a lunar atmosphere, giving the evidence, which may be regarded as conclusive, in proof of the absence of both air and water from the moon, and, therefore, refuting the claim of these elements to be considered as sources or influants of the moon’s volcanic manifestations. A general _coup-d’œil_ of the lunar hemisphere facing the earth next engaged our attention, and we considered the aspect of the disc as it is viewed by the naked eye and with telescopes of various powers. From this general survey we passed to the topography of the moon, tracing briefly the admirable labours of those who have advanced this subject, and, by aid of picture and skeleton maps and a table of position co-ordinates, placing it within the reader’s power to become more than sufficiently acquainted for the purposes of this work with the names and positions of detail objects and features of interest. Special descriptions of interesting and typical spots and regions were given in some few cases where such appeared to be called for.