« НазадПродовжити »
BY DR. BREWSTER.
WHEN we look at the surface of the moon with a good telescope, we find that its appearance is wonderfully diversified. Besides the large dark spots, which are visible to the naked eye, we perceive extensive valleys, and long ridges of highly elevated mountains, projecting their shadows on the plains below. Single mountains occasionally rise to a great height, while bollows, more than three miles deep, and almost exactly circular, are excavated in the plains. The margin of these circular cavities is often elevated a little above the general level, and a high eminence rises in the centre of the cavity. When the moon approaches to her opposition with the sun, the elevations and depressions upon her surface in a great measure disappear, while her disc is marked with a number of brilliant points, and permanent radiations. " It is impossible to imitate," says Mr. Leslie, "the lunar surface, with all its irregular distribution of light and sbade, by a very simple experiment. Introduce a bit of phosphorus into a glass ball of two or three inches in diameter, and, having heated it to catch fire, keep turning the ball round, till half the inner surface being covered with melted phosphorus, the inflamation has ceased. There is left a wbitish crust, or lining, which, in a dark place, will shine for some considerable time. Broad spaces will assume by degrees an obscure aspect, while circular spots, frequently interspersed, will yet glow witb a vivid lustre."
Astronomers have not been content with merely inspecting the surface of the moon, they have even attempted to measure the height of the mountains, and the depth of her cavities ; and though on this point there is a difference of opinion, greater than might have been expected, the results are still highly curious and interesting
Dr. Herschel measured several of the lunar mountains with great care, and found that their height had been greatly over-rated by preceding astronomers. With the exception of a few, it appears that the general height of the mountains does not exceed half a mile.
It may be observed with the aid of a common telescope, that the lunar surface is not only diversified with rocks and cavities, but that some parts of it are distinguished from others by their superi
or illumination. The dark parts of the moon's disc are always smooth, and apparently level ; while the luminous portions are elevated tracts, which either rise into high mountains, or sink into deep and extensive cavities. The general smoothness of the obscure regions naturally induced astronomers to believe that they were immense collections of water. The names given by Hevelius are founded on this opinion; and notwithstanding the discoveries which have been made on the surface of the moon, it is still very generally maintained among modern astronomers. When we examine the moon's disc, however, with minute attention, we find that these obscure portions are not exactly level like a fluid surface. In many of these places, the inequality of surface and of light is considerable; and in some parts parallel ridges are distinctly visible: The large dark spot on the moon's western limb, which is called the Crisian Sea, appears in general to be extremely level; but the writer of this part of the article has often observed, when the moon was a little past her opposition, and when the boundary of light and darkness passed through the Crisian Sea, that this bounding line, instead of being eliptical, as it would have been had the surface been fluid, was irregular, and evidently indicated that this portion of the moon's disc was actually elevated in the middle. The light of these obscure regions, likewise, varies very much, according to the angle of illumination, or the altitude of the sun above their horizon ; and when the moon is near her conjunction, they are not much less luminous than the other parts of ber disc. Now this could never happen if they were covered with water; for when a fluid surface is not ruffled by the wind, the light of the sun, or rather the image of the sun, could not be seen unless when the eye of the observer was in the line of the reflected rays. It would appear, therefore, from these facts, that there is no water in the moon, neither rivers, nor lakes, nor seas; and hence we are entitled to inter, that none of those atmospherical phenomena, which arise from the existence of water in our own globe, will take place in the lunar world.
The strata of mountains, and the insulated bills which mark the disc of this luminary, have evidently no analogy with those in our own globe. Her mountainous scenery, however, bears a stronger resemblance to the towering sublimity, and the terrific ruggedness of Alpine regions, than to the tamer inequalities of less elevated countries. Huge masses of rock rise at once from the plains, and raise their peaked summits to an immense height in the air, wbile
projecting crags spring from their rugged flanks, and, threatening the vallies below, seem to bid defiance to the laws of gravitation. Arouod the base of these frightful eminences are strewed loose and unconnected fragments, which time seems to have detached from their parent mass; and when we examine the rents and ravines which accompany the overhanging cliffs, we expect every moment that they are to be torn from their base, and that the process of destructive separation which we had only contemplated in its effects, is about to be exhibited before us in tremendous reality. The strata of lunar mountains called the Appenines, which traverse a portion of her disc from northeast to southwest, rise with a precipitous and craggy front from the level of the Mare Imbrium. In some places their perpendicular elevation is above four miles; and though they often descend to a much lower level, they present an inaccessible barrier to the northeast, while on the southwest they sink in gentle declivity to the plains.
The analogy between the surface of the earth and moon fails in a still more remarkable degree, when we examine the circular cavities which appear in every part of her disc. Some of these immense caverns are nearly four miles deep and 40 miles in diameter. A high annular ridge generally encircles them; an insulated mountain frequently rises in their centre, and sometimes they contain smaller cavities of the same nature with themselves. These hollows are most numerous in the southwest part of the moon; and it is from this cause that that portion of the moon is more luminous than any other part of her disc. The mountainous ridges, which eacircle the cavities, reflect the greatest quantity of light; and from their lying in every possible direction, they appear near the time of full moon like a number of brilliant radiations, issuing from the large spot called Tycho.
It is difficult to say, with any degree of probability, what these immense cavities have been ; but we cannot help thinking, that our earth would assume the same figure if all the seas and lakes were removed ; and it is therefore probable, that the lunar cavities are either intended for the reception of water, or that they are the beds of lakes and seas which have formerly existed in that luminary. The circumstance of there being no water in the moon, is a strong confirmation of this theory.
The deep caverns, and the broken irregular ground which appear in almost every part of the moon's surface, have induced several astronomers to believe, that these inequalities are of volcanic
origin. This opinion was first maintained by Dr. Hooke in his Micographia, and was afterwards supported by Beccaria, Lichten. berg, aud Æpinus, the latter of whom published a memoir on this subject in 1781 The conjectures of these astronomers have received no small confirmation, from a number of remarkable phenomena which have been seen in the dark part of the moon in the course of the last century. During the annular eclipse of the sun, which happened on the 24th June, 1778, a very singular phenomenon was observed by Don Ulloa. Before the edge of the sun's disc emerged from that of the moon, he observed near the northwest limb of the moon a bright white spot, which he imagined to be the light of the sun shining through an opening in the moon. This phenomenon continued about one minute and a quarter, and was noticed by three different observers. Beccaria observed a spot similar to this in 1772, and imagined that it, as well as that perceiv. ed by Ulloa, were the flames of a burning mountain. Mr. Bode of Berlin also perceived a bright spot in the dark limb of the moon. M. de Villeneuve and M. Nouet saw a luminous point near the spot Heraclides on the 22d May, 1787, and on the 13th March, 1788. It resembled a small nebula, or a star of the 6th magnitude, and seemed to vary considerably in the light which it emitted. This bright spot was again seen on the 8th of May by Mechain, who thought that it was the brilliant point of the spot Aristarchus, shining by the secondary light reflected from the earth. A very brilliant spot was seen in the obscure part of the moon on the 7th March, 1794, by Mr. Wilkins of Norwich, and by Mr. Stretton in London. It appeared in the northeast part of the moon's disc, and continued visible for nearly five minutes. Phenomena of a similar kind have been observed by Dr. Herschel with his usual success. On the 4th May, 1783, he perceived a luminous point in the obscure part of the moon, and two mountains, which were formed from the 4th to the 13th May. In 1787, he perceived similar phenomena, which we shall describe in his own words. " April 19, 1787, 10h. 36' sidereal time. I perceive,” says Dr. Herschel, " three volcanoes in different places of the dark part of the new moon. Two of them are either already nearly extinct, or otherwise in a state of going to break out; which perhaps may be decided next lunation. The third shews an actual eruption of fire, or luminous matter. I measured the distance of the crater from the northern limb of the moon, and found it 3' 57". 3. Its light is much brighter than the nucleus of the comet which M. Mechain discovered at Paris the 10th of this moptb."
ADDRESS TO SWILCAR OAK. Swilcar Oak in Needwood forest, is a very tall tree, measuring thirteen
yards round at its base, and eleven yards round at four feet from the ground, and is believed to be six hundred years old.
BY DR. DARWIN.
Erst, the old Druid-bards with silver hair
Gigantic Oak:—thy hoary head sublime
The poet's verse shall consecrate thy name,
FROM PERCY'S RELIQUES.
DEATH'S FINAL CONQUEST.
Are shadows, not substantial things ;
Scepter and crown
Must tumble down,
And plant fresh laurels where they kill ;
Early or late