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Of suns visible to the naked eye there are about 8000, but the telescope can discern in the Milky ay more than eighteen millions, the number visible increasing as more Clusters of powerful instruments are used. Our cluster of stars. stars is a disc divided into two branches at about one-third of its length. In the midst of innumerable compeers and superiors, the sun is not far from the place of bifurcation, and at about the middle of the thickness. Outside the plane of the Milky Way the appearance would be like a ring, and, still farther off, a nebulous disc. From the contemplation of isolated suns and congre
gated clusters we are led to the stupendous proof matter and blem of the distribution of matter and force
pace. in space, and to the interpretation of those apparent phantoms of self-luminous vapour, circular and elliptic discs, spiral wreaths, rings and fans, whose edges fade doubtfully away, twins and triplets of phosphorescent haze connected together by threads of light and grotesque forms of indescribable complexity. Perhaps in some of these gleaming apparitions we see the genesis, in some the melting away of universes. There is nothing motionless in the sky. In every direction vast transformations are occurring, yet all things proclaim the eternity of matter and the undiminished perpetuity of force.
The theory of gravitation, as delivered by Newton, thus Limit of the
the leads us to a knowledge of the mathematical theory of construction of the solar system, and inferengravitation. tially likewise to that of other systems; but it leaves without explanation a large number of singular facts. It explains the existing conditions of equilibrium of the heavenly bodies, but it tells us nothing of their genesis ; or, at the best, in that particular it falls back on the simple fiat of God.
The facts here referred to conduct us, however, to Phenomena of a
of another and far higher point of view. Some of the solar sys- them, as enumerated by Laplace, are the followtem.
ing:-1. All the planets and their satellites move in ellipses of such small eccentricity that they are nearly circles; 2. The movements of the planets are in the same direction and nearly in the same plane; 3. The movements of the satellites are in the same direction as those of the
acplains the existing tells us nothing of
of the boot, at the cod."
planets; 4. The movements of rotation of these various bodies and of the sun are in the same direction as their orbitual motions, and in planes little different.
The nebular hypothesis requires us to admit that all the ponderable material now constituting the The nebular various bodies of the solar system once extended bypothesis. in a rarefied or nebulous and rotating condition, beyond the confines of the most distant planet. That postulate granted, the structure and present condition of the system may be mathematically deduced.
For, as the vast rotating spheroid lost its heat by radiation, it contracted, and its velocity of rotation was necessarily increased ; and thus were left behind from its equatorial zone, by reason of the centrifugal force, rotating rings, the same result occurring periodically again and again. These rings must lie all in one plane. They might break, collapsing into one rotating spheroid, a planet; or into many, asteroids; or maintain the ringlike form. From the larger of these secondary rotating spheroids other rings might be thrown off, as from the parent mass; these, in their turn breaking and becoming spheroids, constitute satellites, whose movements correspond to those of their primaries.
We might, indeed, advance a step farther, and show how, by the radiation of heat from a motionless nebula, a movement of rotation in a determinate direction could be engendered, and that upon these principles, the existence of a nebulous matter admitted, and the present laws and forces of nature regarded as having been unchanged, the manner of origin of the solar system might be deduced, and all those singular facts previously alluded to explained; and not only so, but there is spontaneously suggested the cause of many minor peculiarities not yet mentioned.
For it follows from the nebular hypothesis that the large planets should rotate rapidly, and the fastes small ones more slowly; that the outer planets ed for by it. and satellites should be larger than the inner ones. Of the satellites of Saturn, the largest is the outermost; of those of Jupiter, the largest is the outermost save one. Of the planets themselves, Jupiter is the largest, and outermost save three. These cannot be coincidences, but must be due to law. The number of satellites of each planet, with the doubtful exception of Venus, might be foreseer, the presence of satellites and their number being determined by the centrifugal force of their primary. The hypothesis also points out the time of revolution of the planets in their orbits, and of the satellites in theirs; it furnishes a reason for the genesis and existence of Saturn's rings, which are indeed its remaining witnesses—their position and movements answering to its requirements. It accounts for the physical state of the sun, and also for the physical state of the earth and moon as indicated by their geology. It is also not without furnishing reasons for the existence of comets as integrant members of our system; for their singular physical state; for the eccentric, almost parabolic orbits of so many of them; for the fact that there are as many of them with a retrograde as with a direct motion; for their more frequent occurrence about the axis of the solar system than in its plane; and for their general antithetical relations to planets.
If there and very many other apparently disconnected Whether ne
facts follow as the mechanical necessities of bulæ actually the admission of a gravitating nebula—a very exist.
simple postulate-it becomes important to ascertain whether, by actual observation, the existence of such material forms may be demonstrated in any part of the universe. It was the actual telescopic observation of such objects that led Herschel to the nebular hypothesis. He concluded that there are two distinct kinds of nebulæ, one consisting of clusters of stars so remote that they could not be discerned individually, but that these may be discerned by sufficient telescopic power; the other being of a hazy nature, and incapable of resolution. Nebulæ do not occur at random in the heavens: the regions poorest in stars are richest in them; they are few in the plane of our sidereal system, but numerous about its poles, in that respect answering to the occurrence of comets in the solar system. The resolution of many of these hazy patches of light into stars by no means disproves the truly nebulous condition of many others.
Fortunately, however, other means than telescopic observation for the settlement of this question are avail. able. In 1846, it was discovered by the author of this book that the spectrum of an ignited solid is continuous, that is, has neither dark nor bright fixed lines. Fraunhofer had previously made known that the spectrum of ignited gases is discontinuous. Here, then, is the means of determining whether the light emitted by a given nebula comes from an incandescent gas, or from a congeries of ignited solids, stars, or suns. If its spectrum be discon. tinuous, it is a true nebula or gas; if continuous, a congeries of stars.
In 1864, Mr. Huggins made this examination in the case of a nebula in the constellation Draco. It proved to be gaseous.
Subsequent observations have shown that of sixty nebulæ examined, nineteen give discontinuous or gaseous spectra; the remainder continuous ones.
It may, therefore, be admitted that physical evidence has at length been obtained, demonstrating the existence of vast masses of matter in a gaseous condition, and at a temperature of incandescence. The hypothesis of Laplace has thus a firm basis.
Notwithstanding the great authority of the astronomers who introduced it, the nebular hypothesis has o encountered much adverse criticism ; not so the nebular much, however, from its obvious scientific defects, by such as its inability to deal with the cases of Uranus and Neptune, as from moral and extraneous considerations. There is a line in Aristophanes which points out precisely the difficulty :
‘O Zeùs oỦK áv, årı’åvri attoll Aivos vuvi Barideówv. A reluctance to acknowledge the presidency of law in the existing constitution and movements of the solar system has been yielded only to be succeeded by a reluctance to acknowledge the presidency of law in its genesis. And yet whoever will reflect on the subject will be drawn to the conclusion that the principle involved was really settled by Newton in his “ Principia”-that is to say, when it became geometrically certain that Kepler's laws originate in a mathematical necessity.
As matters now stand, the nebular hypothesis may be
regarded as the first superficial, and therefore imperfect, glimpse of a series of the grandest problems soon to present themselves for solution-the mathematical distribution of matter and force in space, and the variations of that distribution in time.
Such is the history of the dispute respecting the position of the earth in the universe. Not without reason, therefore,
have I assigned the pontificate of Nicolas V. as The intellec
of the true close of the intellectual dominion of the ecclesias- Church. From that time the sceptre had passed ticism.
into another hand. In all directions Nature was investigated, in all directions new methods of examination were yielding unexpected and beautiful results. On the ruins of its ivy-grown cathedrals, Ecclesiasticism, surprised and blinded by the breaking day, sat solemnly blinking at the light and life about it, absorbed in the recollection of the night that had passed, dreaming of new phantoms and delusions in its wished-for return, and vindictively striking its talons at any derisive assailant who incautiously approached too near. I have not space to describe the scientific activity displayed in all directions; to do it justice would demand volumes. Mathematics, physics, chemistry, anatomy, medicine, and all the many branches of human knowledge received an impulse. Siraultaneously with the great events I have been relating,
every one of these branches was advancing. Wonderful development Vieta made the capital improveinent of using of scientific letters as general symbols in algebra, and applied activity.
that science to geometry. Tycho, emulating Hipparchus of old, made a new catalogue of the stars; he determined that comets are beyond the moon, and that they cut the crystalline firmament of theology in all directions. Gilbert wrote his admirable book on the magnet; Gesner led the way to zoology, taking it up at the point to which the Saracens had continued Aristotle, by the publication of his work on the history of animals; Belon at the same time, 1540, was occupied with fishes and birds. Fallopius and Eustachius, Arantius and Varolius, were immortalizing themselves by their dissections : the former reminding us of the times of Ptolemy Philadelphus, when he naïvely confesses “ the Duke of Tuscany