Зображення сторінки

visible in our most powerful telescopes, rocks and stony masses of great size and weight would be but as the impalpable dust which a sunbeam renders visible as a sheet of light when streaming through a narrow chink into a dark chamber. It is a fact, established by the most indisputable evidence, that stony masses and lumps of iron do occasionally, and indeed by no means unfrequently, fall upon the earth from the higher regions of our atmosphere (where it is obviously impossible they can have been generated), and that they have done so from the earliest times of history. Four instances are recorded of persons being killed by their fall. A block of stone fell at Ægos Potamos, B.C. 465, as large as two mill-stones; another at Narni, in 921, projected, like a rock, four feet above the surface of the river, into which it was seen to fall. The emperor Jehangire had a sword forged from a mass of meteoric iron which fell, in 1620, at Jahlinder, in the Punjab.* Sixteen instances of the fall of stones in the British Isles are well authenticated to have occurred since 1620, one of them in London. In 1803, on the 26th of April, thousands of stones were scattered by the explosion into fragments of a large fiery globe over a region of twenty or thirty square miles around the town of L'Aigle, in Normandy. The fact occurred at mid-day, and the circumstances were officially verified by a commission of the French government.f These, and innumerable other instances t, fully establish the general fact; and after vain attempts to account for it by volcanic projection, either from the earth or the moon, the planetary nature of these bodies seems at length to be almost generally admitted. The heat which they possess when fallen, the igneous phænomena which accompany them, their explosion on arriving within the denser regions of our atmosphere, &c., are all sufficiently accounted for on physical principles, by the condensation of the air before

See the emperor's own very remarkable account of the occurrence, translated in Phil, Trans. 1793, p. 202 + See M. Biot's report in Mém. de l'Institut. 1806.

See a list of upwards of 200, published by Chladni, Annales du Bureau des Longitudes de France, 1825.

them in consequence of their enormous velocity, and by the relations of air in a highly attenuated state to heat.*

(899.) Besides stony and metallic masses, however, it is probable that bodies of very different natures, or at least states of aggregation, are thus circulating round the sun. Shooting stars, often followed by long trains of light, and those great fiery globes, of more rare, but not very uncommon occurrence, which are seen traversing the upper regions of our atmosphere, sometimes leaving trains behind them remaining for many minutes, sometimes bursting with a loud explosion, sometimes becoming quietly extinct, may not unreasonably be presumed to be bodies extraneous to our planet, which only become visible when in the act of grazing the confines of our atmosphere. Among the last mentioned meteors are some which can hardly be supposed solid masses. The remarkable meteor of Aug. 18. 1783, traversed the whole of Europe, from Shetland to Rome, with a velocity of about 30 miles per second, at a height of 50 miles from the surface of the earth, with a light greatly surpassing that of the full moon, and a real diameter of fully half a mile. Yet with these vast dimensions, it changed its form visibly, and at length quietly separated into several distinct bodies, accompanying each other in parallel courses, and each followed by a tail or train.

(900.) There are circumstances in the history of shooting stars, which very strongly corroborate the idea of their extraneous or cosmical origin, and their circulation round the sun in definite orbits. On several occasions they have been observed to appear in unusual, and, indeed, astonishing numbers, so as to convey the idea of a shower of rockets, or of snow-flakes falling, and brilliantly illuminating the whole heavens for hours together, and that not in one locality, but over whole continents and oceans, and even (in one instance) in both hemispheres. Now it is extremely remarkable that, whenever this great display has been exhibited (at least in

* Edinburgh Review, Jan. 1848, p. 195. It is very remarkable that no new chemical element has been detected in any of the numerous meteorolites which have been subjected to analysis.

modern times), it has uniformly happened on the night between the 12th and 13th, or on that between the 13th and 14th of November. Such cases occurred in 1799, 1823, 1832, 1833, and 1834. On tracing back the records of similar phænomena, it has been ascertained, moreover, that more often those identical nights, but sometimes those immediately adjacent, have been, time out of mind, habitually signalized by such exhibitions. Another annually recurring epoch, in which, though far less brilliant, the display of meteors is more certain (for that of November is often interrupted for a great many years), is that of the 10th of August, on which night, and on the 9th and 11th, numerous, large, and bright shooting stars, with trains, are almost sure to be seen. Other epochs of periodic recurrence, less marked than the above, have also been to a certain extent established.

(901.) It is impossible to attribute such a recurrence of identical dates of very remarkable phænomena to accident. Annual periodicity, irrespective of geographical position, refers us at once to the place occupied by the earth in its annual orbit, and leads direct to the conclusion that at that place the earth incurs a liability to frequent encounters or concurrences with a stream of meteors in their progress of circulation round the sun. Let us test this idea by pursuing it into some of its consequences. In the first place then, supposing the earth to plunge, in its yearly circuit, into a uniform ring of innumerable small meteor-planets, of such breadth as would be traversed by it in one or two days; since during this small time the motions, whether of the earth or of each individual meteor, may be taken as uniform and rectilinear, and those of all the latter (at the place and time) parallel, or very nearly so, it will follow that the relative motion of the meteors referred to the earth as at rest, will be also uniform, rectilinear, and parallel. Viewed, therefore, from the center of the earth (or from any point in its circumference, if we neglect the diurnal velocity as very small compared with the annual) they will all appear to diverge from a common point, fixed in relation to the celestial sphere, as if emanating from a sidereal apex (Art. 115.).

(902.) Now this is precisely what actually happens. The meteors of the 12th-14th of November, or at least the vast majority of them, describe apparently arcs of great circles, passing through or near y Leonis. No matter what the situation of that star with respect to the horizon or to its east and west points may be at the time of observation, the paths of the meteors all appear to diverge from that star. On the 9th—11th of August the geometrical fact is the same, the apex only differing; B Camelopardali being for that epoch the point of divergence. As we need not suppose the meteoric ring coincident in its plane with the ecliptic, and as for a ring of meteors we may substitute an elliptic annulus of any reasonable excentricity, so that both the velocity and direction of each meteor may differ. to any extent from the earth's, there is nothing in the great and obvious difference in latitude of these apices at all militating against the conclusion.

(903.) If the meteors be uniformly distributed in such a ring or elliptic annulus, the earth's encounter with them in every revolution will be certain, if it occur once. But if the ring be broken, if it be a succession of groupes revolving in an ellipse in a period not identical with that of the earth, years may pass without a rencontre; and when such happen, they may differ to any extent in their intensity of character, according as richer or poorer groupes have been encountered.

(904.) No other plausible explanation of these highly characteristic features (the annual periodicity, and divergence from a common apex, always alike for each respective epoch) has been even attempted, and accordingly the opinion is generally gaining ground among astronomers that shooting stars belong to their department of science, and great interest is excited in their observation and the further development of their laws. The most connected and systematic series of observations of them, having for their object to trace out their relative paths with respect to the earth, are those of Benzenberg and Brandes, who, by noting the instants and apparent places of appearance and extinction, as well as the precise apparent paths among the stars, of individual meteors, from the extremities of a measured base line nearly 50,000 feet in length, were led to conclude that their heights at the instant of their appearance and disappearance vary from 16 miles to 140, and their relative velocities from 18 to 36 miles per second, velocities so great as clearly to indicate an independent planetary circulation round the sun. [A very remarkable meteor or bolide, which appeared on the 19th August, 1847, was observed at Dieppe and at Paris, with sufficient precision to admit of calculation of the elements of its orbit in absolute space. This calculation has been performed by M. Petit, director of the observatory of Toulouse, and the following hyperbolic elements of its orbit round the sun are stated by him (Astr. Nachr. 701.) as its result; viz., Semimajor axis = -0.3240083 ; excentricity=3.95130; perihelion distance = 0.95626; inclination to plane of the earth's equator, 18° 20'18"; ascending node on the same plane, 10° 34' 48''; motion direct. According to this calculation, the body would have occupied no less than 37340 years in travelling from the distance of the nearest fixed star supposed to have a parallax of 1"].

(905.) It is by no means inconceivable that the earth approaching to such as differ but little from it in direction and velocity, may have attached many of them to it as permanent satellites, and of these there may be some so large, and of such texture and solidity, as to shine by reflected light, and become visible (such, at least, as are very near the earth) for a brief moment, suffering extinction by plunging into the earth's shadow; in other words, undergoing total eclipse. Sir John Lubbock is of opinion that such is the case, and has given geometrical formulæ for calculating their distances from observations of this nature.* The observations of M. Petit would lead us to believe in the existence of at least one such body, revolving round the earth, as a satellite, in about 3 hours 20 minutes, and therefore at a distance equal to 2.513 radii of the earth from its center, or 5000 miles above its surface.

• Phil. Mag., Lond. Ed, Dub, 1848, p. 80.
† Comptes Rendus, Oct. 12. 1846, and Aug. 9. 1847.

« НазадПродовжити »