_Mare Imbrium_, the Sea of Rains here revealed itself in all its vastness to the eyes of the travellers, though it must be acknowledged that the immense depression so called, did not afford them a very clear idea regarding its exact boundaries. Right ahead of them rose _Lambert_ about a mile in height; and further on, more to the left, in the direction of _Oceanus Procellarum_, _Euler_ revealed itself by its glittering radiations. This mountain, of about the same height as _Lambert_, had been the object of very interesting calculations on the part of Schroeter of Erfurt. This keen observer, desirous of inquiring into the probable origin of the lunar mountains, had proposed to himself the following question: Does the volume of the crater appear to be equal to that of the surrounding ramparts? His calculations showing him that this was generally the case, he naturally concluded that these ramparts must therefore have been the product of a single eruption, for successive eruptions of volcanic matter would have disturbed this correlation. _Euler_ alone, he found, to be an exception to this general law, as the volume of its crater appeared to be twice as great as that of the mass surrounding it. It must therefore have been formed by several eruptions in succession, but in that case what had become of the ejected matter? Theories of this nature and all manner of scientific questions were, of course, perfectly permissible to terrestrial astronomers laboring under the disadvantage of imperfect instruments. But Barbican could not think of wasting his time in any speculation of the kind, and now, seeing that his Projectile perceptibly approached the lunar disc, though he despaired of ever reaching it, he was more sanguine than ever of being soon able to discover positively and unquestionably some of the secrets of its formation. [Footnote C: We must again remind our readers that, in our map, though every thing is set down as it appears to the eye not as it is reversed by the telescope, still, for the reason made so clear by Barbican, the right hand side must be the west and the left the east.] CHAPTER XIII. LUNAR LANDSCAPES