310 N E B N E B Ionian king who carried the Jews captive, and whose reign marks the highest point of the Chaldsean empire. Another Biblical form of the word is Nebuchadrezzar (Jer. xlix. 28), and similarly Greek authors write Na/?ov/<oSpo<ropos. These forms are nearer to the original name as it is found on the cuneiform monuments, viz., Nabu-kudurri-usur, " Nebo, defend the crown." To what has been said of Nebuchadnezzar in the article BABYLONIA (vol. iii. p. 188; comp. DANIEL and ISRAEL) it may be added that a frag ment of a cylinder with an inscription relating to a war with Egypt in the thirty-seventh year of his reign has been published by Schrader (Aegypt. Ztschr., 1879; K.A.T., 2d ed., p. 363 s<?.), that an inscription of Nebuchadnezzar has been observed by Sayce on the north bank of Nahr al-Kalb near Beyriit (Proc. Soc. Bib. Arch., 1881), and that two large inscriptions have also been found by Pognon in "Wadi Brissa, near Hermel, in the Lebanon (see Cl. Ganneau in The Times, December 29, 1883). NEBULA. See ASTRONOMY, vol. ii. p. 820. NEBULAR THEORY. The nebular theory is a famous hypothesis which has been advanced with the view of accounting for the origin of the solar system. It is emphatically a speculation ; it cannot be demonstrated by observation or established by mathematical calculation. Yet the boldness and the splendour of the nebular theory have always given it a dignity not usually attached to a doctrine which has so little direct evidence in its favour. It will also be admitted that from the very nature of the case a theory of the origin of the solar system must be devoid of direct testimony. All we could expect to find would be features in that system whose existence the theory would account for ; or possibly by looking at other systems we might observe them in phases suggesting the early phases of our own system. It is hard to see what other kind of evidence would be attainable. Now as a matter of fact our system does present many most striking features which could be accounted for by the nebular theory, and the theory also derives as much corroboration from the study of other systems as we could reasonably expect. Hence, as all attainable evidence is on the whole in favour of the nebular theory (though here and there there are exceptional phenomena), astronomers have generally regarded this theory with considerable approval. There are very remarkable features in the solar system which point unmistakably to some common origin of many of the different bodies which it contains. We must at once put the comets out of view. It does not appear that they bear any testimony on either side of the question. We do not know whether the comets are really indigenous to the solar system or whether they may not be merely imported into the system from the depths of space. Even if the comets be indigenous to the system, they may, as many suppose, be merely ejections from the sun, or in any case their orbits are exposed to such tremendous perturba tions from the planets that it is quite unsafe from the present orbit of a comet to attempt any estimate of what that orbit may have been countless ages ago. On all these grounds we must put the comets on one side for the present, and discuss the nebular theory without any reference thereto. But even with this omission we still muster in the solar system from two to three hundred bodies, almost every one of which pronounces distinctly, though with varying emphasis, in favour of the nebular theory. The first great fact to which we refer is the common direction in which the planets revolve around the sun. This is true not only of the great planets Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune ; it is also true of the host of more than two hundred small planets. All these bodies perform their revolution in the same direction. It is also extremely remarkable that all the great planets and many of the small ones have their orbits very nearly in the same plane, and nearly circular in form. Viewed as a question in prob abilities, we may ask what the chance is that out of two hundred and fifty bodies revolving around the sun all shall be moving in one direction. If the direction of movement were merely decided by chance, the probability against such an arrangement is of stupendous magnitude. It is represented by the ratio of unity to a number containing about sixty figures, and so we are at once forced to the conclusion that this remarkable feature of the planetary motions must have some physical explanation. In a minor degree this conclusion is strengthened by observing the satellites. Discarding those of Uranus, in which the orbits of the satellites are highly inclined to the ecliptic, and in which manifestly some exceptional though unknown influences have been at work, we may say that the satellites revolve around the primaries also in the same direction ; while, to make the picture complete, we find that the planets, so far as they can be observed, rotate on their axes in the same manner. The nebular theory here steps in and offers an explana tion of this most remarkable uniformity. Laplace supposed that our sun had once a stupendous nebulous atmosphere which extended so far out as to fill all the space at present occupied by the planets. This gigantic nebulous mass, of which the sun was only the central and somewhat more condensed portion, is supposed to have a movement of rotation on its axis. There is no difficulty in conceiving how a nebula, quite independently of any internal motion of its parts, shall also have had as a whole a movement of rotation. In fact a little consideration will show from the law of probabilities that it is infinitely probable that such an object should really have some movement of rotation, no matter by what causes the nebula may have originated. As this vast mass cooled it must by the laws of heat have contracted towards the centre, and as it contracted it must, according to a well-known law of dynamics, rotate more rapidly. The time would then come when the centrifugal force on the outer parts of the mass would more than counterbalance the attraction of the centre, and thus we would have the outer parts left as a ring. The inner portion will still continue to contract, the same process will be repeated, and thus a second ring will be formed. We have thus grounds for believing that the original nebula will separate into a series of rings all revolving in the same direction with a central nebulous mass in the interior. The materials of each ring would continue to cool and to contract until they passed from the gaseous to the liquid condition. If the consolidation took place with compara tive uniformity we might then anticipate the formation of a vast multitude of small planets such as those we actually do find in the region between the orbit of Mars and that of Jupiter. More usually, however, the ring might be expected not to be uniform, and therefore to condense in some parts more rapidly than in others. The effect of such contraction would be to draw into a single mass the materials of the ring, and thus we would have a planet formed, while the satellites of that planet would be developed from the still nascent planet in the same way as the planet itself originated from the sun. In this way we account most simply for the uniformity in the direction in which the planets revolve, and for the mutual proximity of the planes in which their orbits are contained. The rotation of the planets on their axes is also explained, for at the time of the first formation of the planet it must have participated in the rotation of the whole nebula, and by the subsequent contraction of the planet the speed with which the rotation was performed must have been accelerated.
