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and never obscured by clouds, its features are wholly open to view, and the eye, aided with glasses, may wander over its rugged crags, survey its craters, its Alps and its Apennines, from their bases to their summits. Neither are there any sedimentary deposits, soil or vegetation,--for there can be none without water,-and the igneous surface, therefore, is still its own naked self, exhibiting the results of igneous action in their simple grandeur, unaltered and uncomplicated by any attending operations. We may hope, therefore, to find some profit in contemplating for a few moments this land of the skies: and although we may not look for very speedy "annexation," we may possibly gather some facts and ideas which the decree of Truth will annex to the domain of Science.

The moon, as we all know, has been minutely studied in a physical point of view, and already some important geological conclusions have been drawn from the facts it presents. The

eclipse, Sept. 5, 1793, observed the sharp horn of the limb of the moon, and says that it seemed perfectly regular; and that a deviation of a single second by the refraction of the solar light in the moon's atmosphere would not have escaped him. Phil. Trans., 1794, p. 39.-As stated in Beer and Mädler (p. 133), Schröter calculated the density of the supposed atmosphere to be one twenty-eighth the density of our own atmosphere; and Melanderhjelm demonstrated that the moon's atmosphere, judging from that of the earth, should have one thirty-sixth the density of our own atmosphere. But the above-mentioned authors say that we have yet to prove that the moon has any atmosphere, adding, that it must be very much more rare than the rarest gas on earth. They observe also, that supposing our atmosphere to extend through space, its density half way to the moon, according to the Mariottian law of decrease, would be expressed by the fraction 100000, the denominator extending to ten thousand zeros. The singular observation occasionally made, that during the passage of the moon over a star, the star appears visible in front of the edge of the moon, before disappearing, may possibly indicate an extremely thin atmosphere or surface vapours: but it has been attributed, with much appearance of reason (Rep. Brit. Assoc., 1845, p. 5), to diffraction.

The absence of any bodies of water on the moon is placed beyond doubt, both by actual telescopic examination, and by inference from the absence of clouds. There are no streams, lakes, or seas. An eminent astronomer has remarked, that the heat of the surface exposed to the sun would occasion a transfer of any water the moon might contain to its dark side, and that there may be frosts in this part, and perhaps running water near the margin of the illuminated portion. But in such a case, would not clouds appear about the margin at times in telescopic views?

altitudes of its mountains were first estimated by Galileo,* and afterwards were mathematically calculated by Heveliust and Riccioli. Sir William Herschel continued the investigations, and reported the probable activity of three of its volcanic mountains. Mayer, Huth, Harding, and Schröter,§ and more lately Gruithuisen and W. G. Lohrmann,|| are other prominent names among those who have added largely to our knowledge of the moon's surface. More recently still, MM. Beer and Mädler have pursued this science of Selenography with wonderful perseverance and labour, and have given corrected results of all previous calculations, with magnificent maps of the moon's topography. 1095 heights were carefully measured by them, and their features, to a great degree of accuracy, ascertained. These maps have afforded M. Elie de Beaumont some deductions alleged as supporting certain geological theories. James Nasmyth, Esq., in the Transactions of the Royal Astronomical Society for the present year, has published important observations on the features of the moon's mountains, and traced out their volcanic character.** A very valuable memoir, on the same topics, has been presented within the current year to

*In the article referred to in the Annuaire des Longitudes, (p. 522), Arago states that Clearchus, on the authority of Plutarch, described the moon as smooth and lustrous like a mirror. Democritus attributed the spots to inequalities of surface. Galileo first observed the lunar mountains with his telescope in 1610, and estimated their height at one-twentieth of the diameter, giving 8800 metres for their altitude, which but little exceeds their actual height.

† J. Hevelius, Selenographia; fol., Gedani, 1647.

Phil. Trans. for 1780, p. 507, Astronomical Observations relating to the Moon :-for 1787, p. 229, An Account of three Volcanoes in the Moon:-for 1794, p. 39, Account of some particulars observed during the late eclipse (in 1793) of the Sun.

§ J. H. Schröter, Selenotopographische Fragmente zur genauren Kenntniss der Mondfläche ihrer erlittenen Veränderungen und Atmosphäre; 2 vols, 4to, Göttingen, 1791 and 1802.-Gruithuisen, in Bode's Astron. Jahrb., 1825.

Topographie der sichtbaren Mondoberfläche, von W. G. Lohrmann; 4to, Dresden und Leipzig, 1824.

Allgemeine vergleichende Selenographie; mit besonderer Beziehung auf die von den Verfassern herausgegebene Mappa selenographica, von W. Beer und Dr J. G. Mädler; Berlin, 1837.

** Memoirs of the Royal Astronomical Society, vol. xv., 1846. On the Telescopic Appearance of the Moon, by James Nasmyth, Esq., p. 147.

the Institute at Paris, by M. Rozet, in which the moon is shewn to have been a globe in complete fusion, which has slowly cooled; and its peculiarities are dwelt upon as an exhibition, in many respects, of the former state of our own planet.*

In all the geological observations which have been hitherto made with regard to the moon, one important feature remains unsatisfactorily explained. I refer to the vast magnitude of its craters. It is not surprising that, in view of their stupendous size, many should have been incredulous as to their crater character, and preferred to designate them by some non-committal term, as circular ridges, or ring-mountains; nor that geologists, in general, have hardly ventured to acknowledge their belief in these lunar wonders. Imagine, if possible, in place of an ordinary crater, circular areas 50 to 150 miles in diameter, and 10,000 to 20,000 feet in depth. Such are many of the lunar craters; and they are crowded in great numbers over the larger part of its surface, varying from even a more capacious magnitude, down to those that measure but a few miles in breadth. It is not astonishing that there should be found much difficulty in reconciling their features with those of Vesuvius and Etna, hitherto received too generally as the types of volcanoes and volcanic action. The crater of Kilauea in the Hawaiian Islands is of a wholly different character, and I propose to present some illustrations which it affords, appealing to such general facts regarding it as are already well known. If I mistake not, it will be found to give a full interpretation of whatever has been considered mysterious in these lunar ring-mountains. After these illustrations, we may return again to earth, and apply the knowledge which we have derived abroad, in exemplifying the former geological history of our own planet. We may first consider the general features of the moon's surface.

About two-thirds of the lunar hemisphere in view, comprising almost the whole of the southern half and the northeast quarter, are covered thickly with volcanic mountains.

* Sur la Sélénologie, by M. Rozet, Comptes Rendus, 1846, xxii., 470.

Over a large part of the north-west quarter there is only here and there an elevation, and this comparative nudity extends a considerable distance southward across the equator. The features of the surface may be distinguished as of five kinds, viz:

1. The ring-mountains, which are broad truncated cones with immense circular craters. (See the following figures from Beer and Mädler.)

2. Conical mountains, nearly like ordinary volcanoes. 3. Linear or irregular ridges.

4. Large depressed areas, usually termed seas, but not supposed to contain water.

5. Broad pale streaks, of great length.

6. Narrow lines, supposed to be fissures.

Out of the 1095 heights measured by Beer and Mädler, six are above 20,000 feet in altitude, and twenty-two. exceed 15,750 feet.

The broad truncated cones with large circular craters, are its most common elevations, and are among the loftiest. The pits, as we have remarked, are of all dimensions to 150 miles, and of various depths to near 25,000 feet. The crater Baily is 149 statute miles in diameter; Clavius is 1433 miles ; Schickard is 128 miles. 20 to 60 miles is the more common breadth. The depth of Newton is 23,833 English feet; of Casatus 22,822; of Calippus 22,209; of Tycho 20,181 feet. The height above the surface exterior to the cone, is

*

* We have stated that Galileo (note to page 336) made the altitude of the higher of the moon's mountain 8800 metres. Hevelius reduced their height to 5200 metres. Riccioli, as M. Arago states, increased Galileo's estimate, and his observations, as calculated by M. Keill, gave for the mountain St Catherine more than 14,000 metres. Herschel in 1780 (Phil. Trans. for 1780, p. 507; also for 1794, p. 40), reduced again the heights, concluding, from his observations, that the loftiest did not exceed a mile and a half. The latest investigations have restored them nearly to Galileo's first estimate.

We state, for the information of some who have not paid attention to the subject, that these heights are determined, either from the shadows of the peaks on the central plain of a crater or the exterior surface, or by noting the position of a summit when it first becomes illuminated, and calculating therefrom; the higher the peak, the longer will be the shadow, and also, the sooner its top will be tipped with light. Should it hereafter be established that the moon has an atmosphere, it must be too slight to affect appreciably the altitudes de

said by Beer and Mädler to be often but one-half or one-third the height above the bottom of the crater; the outer slopes

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termined with regard to the breadth of the craters, there can be no more doubt, than with respect to the diameter of the moon itself.

sume.

There are many who receive with scepticism the facts we have stated, or even deny where they know nothing. It is taking a high ground, to dispute with all the astronomers since Galileo, and one to which ignorance alone would preThe best antidote we can propose to such presumption, is to take the first opportunity which offers, to look through a good telescope at the moon's surface, and examine its features for themselves. We predict that they will soon become conscious of a growing willingness to be humble learners of such men as Herschel and others who have made the moon their study.

* Mem. of the Roy. Astronom. Soc., xv., 152.

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