soi.AK SYSTEM. 





the nebular theory u a primitirt caiitt, it ia true that ho did no in hia 

 own WDM f the words. Mr. Whcwell mean* by ]>riiuitive cnuso a 

 first muse, at those words are usually understood ; and he aalu*(and 

 the question would have been much to the purpose if Laplace had 

 really meant the same thing as himself by the words primitirt caiue), 

 " Was man, with Ilia thought and feeling, his powers and hopes, hia 

 will and conscience, also produced as an ultimate result of the con- 

 densation of the solar atmosphere f" But Laplace speaks as follows: 

 " Quclle est ct'tte came primitirt t J'exposerai sur celh, dans la note 

 qui U-nnine cett ouvrage, unc hypothese," Ac. And in the very first 

 words of this note we rind, " On a, pour remonter K la cautt dea 

 mouvemens prmitift du systcme plnnctaire," &c. This then is what 

 Laplace understood by primilnr taint, a tame of the primitive 

 motions ; an improper use of language, if the reader pleases; but 

 when a man puts hia own meaning on his own words, no one has a 

 right to fix the consequences of another meaning upon him. 



We now proceed to the nebular theory, which ia a conjecture pro- 

 posed with much doubt by Laplace, as a possible explanation of the 

 manner in which the morions of the several planets obtained those 

 remarkable resemblances which are found to subsist, without making 

 the inquiry extend to anything except their motions. All the planeta 

 move in one direction round the sun, and their satellites move in the 

 same direction round themselves ; those that are known to revolve 

 round their axes (and the contrary has been proved of no one of them) 

 also revolve in the same direction, and their equators are not much 

 inclined to their orbits. The cxcentricities of the planets and satellites 

 are in no case very large, and generally very small ; and the inclinations 

 of their orbits to one another are generally small. Many nebulae in the 

 heavens appear, when examined, to consist of a bright nucleus sur- 

 rounded by nebulous matter ; in others it is found that the apparently 

 nebulous matter consists of stars. This gave Laplace the idea that our 

 System might originally that is, previously to the establishment of 

 its present order have been a large nebula, of which the sun was at 

 the centre. Imagine a large nebulous mass in a state of revolution, 

 with a solid, or at least less nebulous, centre, round which it revolves : 

 call this ccntr.il nucleus the sun. Assume the ordinary laws of matter to 

 be true of this nebulous mass ; and also that it extends as far as such 

 an atmosphere can do namely, until the attraction of the whole upon 

 particles at the equator is equal to the centrifugal force of those 

 particles. If condensation should begin to take place, arising from loss 

 of heat, the moss would revolve more and more rapidly as it was 

 condensed into less and less space ; but it does not follow that the 

 equatorial particles would fall in towards the centre : they are balanced 

 by the equality of the centripetal and centrifugal forces, and might 

 form a ring round the rest of the mass. If the process were conducted 

 with great regularity, this ring and the mass of vapour might undergo 

 continual condensation together, until the increasing velocity of rotation 

 prevented the formation of the ring from continuing. The departures 

 from complete regularity which might exist in the moss might cause 

 disturbances in the formation of the rings, which might end in there 

 being one or more (not many) permanently revolving round the rest of 

 the mass condensed into a solid body, in the manner of Saturn and its 

 rings. Such regular formation, however, might be rarely continued 

 long enough ; and if the rings got broken, each ring would become 

 several masses, which would revolve nearly at the same distance, and 

 nearly with the same velocity : such a result is seen in the four small 

 planets. But as, generally speaking, these masses would, by irregu- 

 larities in their velocities, be combined into one * at last, each broken 

 ring would form a new nebulous mass, revolving round the diminished 

 central nucleus; and if a number of such masses were formed, those 

 nearer to the central moss would move with the greater velocity, and 

 would be both smaller and denser than the external ones : the first 

 circumstance certainly, the second and third most probably. Again, 

 each mass would have a motion of revolution in the same direction 

 [MOTION, DIRECTION OF] as the motion round the primary ; for when 

 the ring becomes broken, its internal parts have a somewhat more 

 rapid motion than the external ones, which would give the motion of 

 rotation noticed. And the rotations thus created in the internal 

 musses would probably be greater than those in the external masses. 

 The orbits of the masses would necessarily be nearly circular, and not 

 much inclined to each other ; but for irregularities, quite circular, and 

 in the same plane. In each of the nebulous masses thus detached and 

 revolving, condensation might again give rings or satellites, or both ; 

 but in all probability the external masses would get more satellites 

 than the internal ones : the orbits of the satellites must be also nearly 

 circular, and not much inclined. All the preceding circumstances, 

 both those which are certain and those which are probable (Laplace 

 confined himself to the former), are actually existing in the s,i| ; ,i- 

 System ; consequently this hypothesis, though subject to serious diffi- 

 culties, deserves attentive consideration, as often as any now knowledge 

 of the constituent parts of our system shall render a reference to it 

 likely to produce evidence on one side or the other. As a substitute 



If any number of mnv, capable of cohering, revolve in orbits ao near to 

 one another, that they must cohere when thcjr come to their minimum distance, 

 nothing but an abwlute and mathematical equality in their mean relocitic 

 can keep them permanent!}- asunder ; the amallcst inequality must at laat bring 

 them all together. 



for intelligent creative power, if such a thing were intended, it would 

 dn no better tlxan any other; for, as Mr. Whewell observes, a man 

 with will, | lower, and conscience, cannot be admitted to be a necessary 

 consequence of the cooling of a nebulous atmosphere. Nevertheless, 

 as exhibiting a possible mode in which the Creator of mind and matter 

 made the laws of formation resemble those of continuation, as far as 

 the motions of the system are concerned, this hypothesis is strikingly 

 explicative of what we really see. But even if we were to take it to 

 be a true explanation, it would only be one step of the ascent, and the 

 next question would be, whaf higher process distributed the parts of 

 this nebulous mass in such a manner as to place those outermost which 

 were fit to form a planet so distant from the source of light and heat 

 OH Neptune, and to support the appropriate forms of animal and 

 vegetable life which analogy would induce us to suppose must exist 

 there. 



The history of astronomy teaches us that the system in which wo 

 live has not undergone any apparent change for more than 2000 years ; 

 and, on inquiring into the connection which exists between one pi. met 

 and another, or the laws of gravitation, it is found that so far as their 

 mutual actions are concerned, there is no reason why any change ever 

 shculd take place. If the central body were the only one which 

 attracted the rest, and as long as the laws of matter remained unal- 

 tered, it is certain that nothing could alter the revolutions of a system 

 of planets, unless two orbits intersected, and the planets of those orbitg 

 happened to come to the intersecting part at the same time, and to 

 strike each other. But the planets are subject to the action of each 

 other as well as to that of the sun, and no instant elapses without 

 every orbit undergoing a slight change from every one of the planets 

 of the other orbits. Jupiter alone produces on the earth's orbit in one 

 year more change than we have any right to say all the comets put 

 together would do in a hundred. And yet the system nut only con- 

 tinues without any sensible change, but, one circumstance alone ex- 

 cepted, to which we shall presently allude, is demonstrably forme I to 

 continue for a most enormous length of time, unless some new action 

 should arise, or some external cause begin to operate. As it is some- 

 times stated that a complete mathematical demonstration has been 

 given of the eternal stability of the Solar System, so far as the mutual 

 actions of its parts are concerned an assertion which is altogether 

 incorrect it may be worth while to enter a little on the details of 

 this subject. 



The disturbing forces of the planets on each other cannot have their 

 effects calculated all at once ; but each force must be divided into an 

 infinite series of terms, the first of which contains all the terms of the 

 first dimension, the second all those of the second, and so on. Of all 

 these terms each is much less in its effect than the preceding ; so that 

 in fact the first two dimensions are all that produce any sensible effect 

 in any time which it is worth while to consider. Occasionally it 

 happens that terms of the third and fourth dimensions have been 

 required to be used, but almost all the sensible perturbations of the 

 system depend on terms of the first two orders. As far as any effects 

 arising from such terms are concerned, Lagrange and Poisson are ad- 

 mitted on all hands to have demonstrated the stability of the Solar 

 System: and considering the nature of the process employed, and 

 there being no appearance of any circumstance which looks likely to 

 lead to a different result in any of the remaining terms of the dis- 

 turbing forces, it may be highly probable that a further investigation 

 would show the same thing, if all the dimensions of the disturbing 

 forces were employed. Sir J. Lubbock (' Phil. Mag.,' February, 1831) 

 has pointed out the forms which further investigation would appa- 

 rently produce, and which would (unless a detailed investigation should 

 lead to something not discoverable d, priori) bear out as certain what 

 we have just stated to be probable. But though all the presumptions 

 lie on the side of those who would assert the proposition absolutely 

 of all dimensions of the disturbing force, it is not yet time to say that 

 it is a certain mathematical consequence of the theory of gravitation. 



When the effects of perturbation are examined, as for as the second 

 dimension of the disturbing force, it appears that the immense 

 mass of the sun compared with that of any planet, the great distance 

 of the planets from each other as compared with their amounts of 

 departure from spherical form, the small exccntricities and inclinations 

 of their orbits, and their motions being all in one direction, give the 

 following mathematical consequences of the law of attraction : First, 

 the longest or major axes of the planets' orbits are not subject to any 

 K 1. >w variations of very long period ; all their variations being excessively 

 small, and soon destroyed by the production of contrary variations. It 

 is very often stated that the major axes are subject to no wiatioo ; 

 this is to be understood only of lectUar variation (or of a very long 

 period). One year U not precisely the same as another to any fraction 

 of a second ; but the average year of one long period w precisely the 

 same as that of another ; or at least the mean years of the two periods 

 become more nearly equal the longer the periods are made. But the 

 exccntricities and inclinations are subject to long periodic altera- 

 tions, the times of their recurrences not being exactly settled, from 

 the difficulty of their determination. How then is it known that 

 they ore periodic ? For instance, the exoentricity of the Earth's orbit 

 is subject to a yearly diminution of "00004, its value in 1801 being 

 017. Had this diminution been an increase, as it is in Mercury 

 and Jupiter, it might A privri ,-I]>IHMI- possible that this increase 



