well-defined hill, close to which is a crater-like depression, and other less considerable hills. (r) Analogy to terrestrial volcanoes more apparent than real — enlarged view of Gassendi.-The volcanic character observed in the selenographic formation loses much of its analogy to like formations on the earth's surface when higher magnifying powers enable us to examine the details of what appear to be craters, and to compare their dimensions with even the most extensive terrestrial craters. Numerous examples may be produced to illustrate this. We have seen that Tycho, which, viewed under a moderate magnifying power, appears to possess in so eminent a degree the volcanic character, is, in fact, a circular chain enclosing an area upwards of fifty miles in diameter. Gassendi, another system of like form, and of still more stupendous dimensions, is delineated in Plate XIII, as seen with high magnifying powers. This remarkable object consists of two enormous circular chains of mountains, the lesser, which lies to the north, measuring 16 miles in diameter, and the greater, lying to the south, enclosing an area 60 miles in diameter. The area enclosed by the former is therefore 214, and by the latter 2827 square miles. The height of the lesser chain is about 10,000 feet, while that of the greater varies from 3500 to 5000 feet. The vast area thus enclosed by the greater chain includes, at or near its centre, a principal central mountain, having eight peaks and an height of 2000 feet, while scattered over the surrounding enclosure upwards of a hundred mountains of less considerable elevation have been counted. It is easy to see how little analogy to a terrestrial volcanic crater is presented by these characters. The preceding selections, combined with the charts, Plates I., XI., XII., and XIII., will serve to show the general physical character of the lunar surface, and the elaborate accuracy with which it has been submitted to telescopic examination. In the work of Beer and Mädler a table of the heights of above 1000 mountains is given, several of which attain to an elevation of 23,000 feet, equal to that of the highest summits of terrestrial mountains, while the diameter of the moon is little more than a quarter that of the earth. 213. Observations of Herschel.-Sir John Herschel says, that among the lunar mountains may be observed in its highest perfection the true volcanic character, as seen in the crater of Vesuvius and elsewhere; but with the remarkable peculiarity that the bottoms of many of the craters are very deeply depressed below the general surface of the moon, the internal depth being in many cases two or three times the external height. In some cases, he thinks, decisive marks of volcanic stratification, arising from a succession of deposits of ejected matter, and evident indications of currents of lava streaming outwards in all directions, may be clearly traced with powerful telescopes. 214. Observations of the Earl of Rosse. - By means of the great reflecting telescope of Lord Rosse, the flat bottom of the crater called Albategnius is distinctly seen to be strewed with blocks, not visible with less powerful instruments; while the exterior of another (Aristillus) is intersected with deep gullies radiating from its centre. 215. Supposed influence of the moon on the weather. Among the many influences which the moon is supposed, by the world in general, to exercise upon our globe, one of those, which has been most universally believed, in all ages and in all countries, is that which it is presumed to exert upon the changes of the weather. Although the particular detai's of this influence are sometimes pretended to be described, the only general principle, or rule, which prevails with the world in general is, that a change of weather may be looked for at the epochs of new and full moon: that is to say, if the weather be previously fair it will become foul, and if foul will become fair. Similar changes are also, sometimes, though not so confidently, looked for at the epochs of the quarters. A question of this kind may be regarded either as a question of science, or a question of fact. If it be regarded as a question of science, we are called upon to explain how and by what property of matter, or what law of nature or attraction, the moon, at a distance of a quarter of a million of miles, combining its effects with the sun, at four hundred times that distance, can produce those alleged changes. To this it may be readily answered that no known law or principle has hitherto explained any such phenomena. The moon and sun must, doubtless, affect the ocean of air which surrounds the globe, as they affect the ocean of water-producing effects analogous to tides; but when the quantity of such an effect is estimated, it is proved to be such as could by no means account for the meteorological changes here adverted to. But in conducting investigations of this kind we proceed altogether in the wrong direction, and begin at the wrong end, when we commence with the investigation of the physical cause of the supposed phenomena. Our first business is carefully and accurately to observe the phenomena of the changes of the weather, and then to put them in juxtaposition with the contemporaneous changes of the lunar phases. If there be any discoverable correspondence, it then becomes a question of physics to assign its cause. Such a course of observation has been made in various observatories with all the rigour and exactitude necessary in such an inquiry, and has been continued over periods of time so extended, as to efface all conceivable effects of accidental irregularities. We can imagine, placed in two parallel columns, in juxtaposition, the series of epochs of the new and full moons, and the quarters, and the corresponding conditions of the weather at these times, for fifty or one hundred years back, so that we may be enabled to ex amine, as a mere matter of fact, the conditions of the weather for one thousand or twelve hundred full and new moons and quarters. From such a mode of observation and inquiry, it has resulted conclusively that the popular notions concerning the influence of the lunar phases on the weather have no foundation in theory, and no correspondence with observed facts. That the moon, by her gravitation, exerts an attraction on our atmosphere cannot be doubted; but the effects which that attraction would produce upon the weather are not in accordance with observed phenomena; and, therefore, these effects are either too small in amount to be appreciable in the actual state of meteorological instruments, or they are obliterated by other more powerful causes, from which hitherto they have not been eliminated. It appears, however, by some series of observations, not yet confirmed or continued through a sufficient period of time, that a slight correspondence may be discovered between the periods of rain and the phases of the moon, indicating a very feebie influence, depending on the relative position of that luminary to the sun, but having no discoverable relation to the lunar attraction. This is not without interest as a subject of scientific inquiry, and is entitled to the attention of meteorologists; but its influence is so feeble that it is altogether destitute of popular interest as a weather prognostic. It may, therefore, be stated that, as far as observation combined with theory has afforded any means of knowledge, there are no grounds for the prognostications of weather erroneously supposed to be derived from the influence of the sun and moon. Those who are impressed with the feeling that an opinion so universally entertained even in countries remote from each other, as that which presumes an influence of the moon over the changes of the weather, will do well to remember that against that opinion we have not here opposed mere theory. Nay, we have abandoned for the occasion the support that science might afford, and the light it might shed on the negative of this question, and have dealt with it as a mere question of fact. It matters little, so far as this question is concerned, in what manner the moon and sun may produce an effect on the weather, nor even whether they be active causes in producing such effect at all. The point, and the only point of importance, is, whether, regarded as a mere matter of fact, any correspondence between the changes of the moon and those of the weather exists? And a short examination of the recorded facts proves that IT DOES NOT. 216. Other supposed lunar influences.—But meteorological phenomena are not the only effects imputed to our satellite: that body, like comets, is made responsible for a vast variety of interferences with organised nature. The circulation of the juices of vegetables, the qualities of grain, the fate of the vintage, are all laid to its account; and timber must be felled, the harvest cut down and gathered in, and the juice of the grape expressed, at times and under circumstances regulated by the aspects of the moon, if excellence be hoped for in these products of the soil. According to popular belief, our satellite also presides over human maladies; and the phenomena of the sick chamber are governed by the lunar phases; nay, the very marrow of our bones, and the weight of our bodies, suffer increase or diminution by its influence. Nor is its imputed power confined to physical or organic effects; it notoriously governs mental derangement. If these opinions respecting lunar influences were limited to particular countries, they would be less entitled to serious consideration; but it is a curious fact that many of them prevail and have prevailed in quarters of the earth so distant and unconnected, that it is difficult to imagine the same error to have proceeded from the same source. Our limits, and the objects to which this volume is directed, render it impossible here to notice more fully the physical and · physiological influences imputed to the moon, more especially as these influences, though interesting in themselves, have but an indirect connection with lunar astronomy. 217. The lunar theory.—This important branch of theoretical astronomy is far too advanced for these pages, as of all the planetary movements, those of the moon are the most complicated. In a maritime country like England, whose seamen have to depend so much on lunar observations at sea, the subject is of the highest importance, and the observation of the moon is therefore one of the principal daily duties at the Royal Observatory. Since the publication of the results of the lunar observations made there since 1750, some of the greatest mathematical astronomers have reinvestigated the peculiar movements of the moon, with the object of improving the lunar theory. Among them may be mentioned Professor Hansen of Gotha, Professor Adams of Cambridge, M. Delaunay of Paris, and Sir George Airy, the Astronomer Royal. From the published theory of Professor Hansen, lunar tables have been formed and adopted in the calculations of the Nautical Almanac, and they are considered very far in advance of the tables of Burckhardt which they superseded, although Hansen's lunar theory is far from being perfect. M. Delaunay's researches on the theory in the most important sections were completed at the time of his lamented death, but certain portions of his work remaining unfinished have prevented his theory from assuming the practical form of tables, and it is not therefore available for easy comparison with the observations. |