THE STELLAR UNIVERSE. 



395 



but a moment in the growth of a system. The length of some astronomical 

 periods, the reality of which is not disputed, is adduced to justify this. The 

 planet Herschel or Uranus, has not yet completed a single period since its 

 discovery; and several of the binary stars have been observed to move 

 through only a small arc of their entire course. In the solar system many 

 secular changes have been discovered, the completion of which will occupy 

 many thousand years. Yet the reality of these is not the less certain. It is 

 contended, therefore, that the gradual change of a nebula into a system, not 

 having been actually seen, is no conclusive argument against its possible ex- 

 istence. 



In the celebrated nebular hypothesis, which its illustrious author propounds 

 as a mere conjecture, and with great diffidence it is supposed — that the sun 

 has been formed by the gradual condensation and solidification of a mass of 

 nebulous matter; that it revolved together with the nebulous atmosphere 

 around it in the direction in which the planets now revolve, and which atmo- 

 sphere, by the operation of an excessive degree of heat, extended to a distance 

 from the common centre much greater than that of the most remote planet ; 

 that as this heat gradually diminished, the solar atmosphere contracted accord- 

 ing to the common law of cooling bodies ; that in accordance with the laws 

 of revolving bodies, the velocity o£ its rotation increased ; that an exterior 

 zone of vapor was detached from the rest, where the centrifugal force pro- 

 duced by the central motion, exceeded the central attraction ; that such zone 

 of vapor might collect into a ring like those of Saturn ; might aggregate into 

 several masses revolving nearly in the same circle, like the new planets ; or, 

 finally, might coalesce into a single mass. Thus would be formed a number 

 of planets which at first would be vapor. These planets would, according to 

 the laws of mechanics, have rotatory motions on their axes ; these rotatory 

 motions would be all in the same direction, and as the vapor would gradually 

 cool down, each planet might form round its own centre satellites or rings, in 

 the same manner as the planets themselves would be formed round the central 

 sun. 



This supposition will evidently explain the most obvious provisions of our 

 system. If it did not, it would never have been proposed by its author. It is 

 evident that all the principal motions of such a system would be nearly circu- 

 lar, and nearly in the plane of the original motion of rotation of the nebulous 

 mass. It is easily proved also that the motions of the satellites round the 

 planets respectively, and the motion of both planets and satellites on their 

 axes would be in one common direction and one common plane. Thus it 

 may be admitted that the most important laws on which the stability of the 

 solar system depends would be explained. 



But some modern writers on this subject have ascribed to this conjecture of 

 a distinguished man a much more serious character than the author himself 

 claimed for it. Laplace too well understood the rigorous canons of inductive 

 philosophy to view his guesses as anything higher than an extremely refined 

 and ingenious conjecture, certainly not deserving the name of a theory, and 

 scarcely proper to be called even an hypothesis. 



It is not worth while here to notice the innumerable arrangements which it 

 fails to explain, arrangements certainly not less important than those which 

 have been selected as the basis of the conjecture. But it may be well to warn 

 those who are little familiar with such inquiries against the errors to which such 

 an hypothesis might give birth. It is the general tendency of every ascent 

 in the analysis of causation to give the appearance of superseding the supposi- 

 tion of an omnipotent agent above matter and its laws, the (ountain of the intel- 

 ligence, wisdom, design, and beneficence, manifested in the visible creation. 



