Page:The American Cyclopædia (1879) Volume XII.djvu/210

202 NEBULAE HYPOTHESIS concentration, but before it has begun to take a liquid or solid form, the following actions will go on: 1, more and more rapid aggrega- tion of its atoms into a smaller and denser mass, as the common centre of gravity is ap- proached; 2, development of oblateness; 3, evolution of heat, greatest at the central parts; and, as a consequence, 4, circulation currents setting from the centre toward the poles and thence to the equator, and counter currents from the equator to the centre. In the course of this round there will be, 5, an oscillation of temperature : first, from the centre outward expansion by diminished pressure and other causes, and consequent lowering of tempera- ture ; secondly, from the equator inward rise in temperature for converse reasons. 6. As a corollary to 4 and 5, external condensation will occur according to the laws of precipita- tion from gases, resulting in a belt of vapor about the equator, gradually widening and condensing into a fluid ; 7, this fluid film will gradually extend itself till it eventually closes over at the poles, thus forming a thin hollow spheroid filled with gaseous matter; 8, at length the liquid shell will become very thick, the outer surface will experience a fall of tem- perature and begin to harden into a solid crust. This hypothesis explains the relative specific gravities of the planetary bodies, the formation of the asteroids, the earth's sup- posed interior structure, indications of past or present high temperature throughout the solar system, and the sun's incandescence. These considerations relate chiefly to the physical changes undergone by a forming system. Laplace's nebular hypothesis deals with the changes of arrangement in the distribution of matter forming into a system under the action of dynamical laws. He takes as the basis of his theory certain features of our solar sys- tem which are not explained by the theory of gravitation. Gravity accounts for Kepler's laws, which are shown to be among its neces- sary consequences. No system could circulate in any manner around a centre, for instance, without the law holding that the numbers representing the cubes of the mean distances would be proportional to the numbers repre- senting the squares of the periodic times. But a system could exist under gravity in which the planets would travel in widely eccentric orbits or in planes largely inclined to each other. Nor has it been proved that the plan- ets might not safely circulate in different di- rections. Assuredly, if revolution in different directions, or in planes largely inclined to each other, or in very eccentric paths, might in the long run result in collisions and therefore in the destruction of the system as such, there is yet no reason to believe that all the axial ro- tations need take place in the same direction as the motions of revolution. But, to say the truth, none of those laws of harmony in our solar system, except the laws depending di- rectly on gravity, can be regarded as essential to the well being of the system ; nor, as will presently appear, would the difficulty of re- garding the system as other than a product of evolution be appreciably diminished by sup- posing that without those laws the destruction of the system must inevitably have occurred in the course of time. For it would be mani- festly unreasonable to regard our system as one in which the original arrangements were fortuitously so happy that it has continued to exist as a system, if we find that the proba- bility of these arrangements so existing by mere coincidence is exceedingly minute. Now, how small this probability is may be inferred by considering only the motion of the planets in one common direction. There are known at the present time 8 major planets and 137 minor planets (the number of these is increas- ing year by year). Thus there are 145 known planets. Taking the earth's direction of revo- lution as a standard direction, the chance that any one of the remaining 144 planets would have this direction as a result of mere chance is of course one half, since a planet must re- volve in one of two ways. Therefore, by the laws of probability, the chance that all the 144 other planets would revolve in that direction is represented by a fraction whose numerator is unity and its denominator 2 raised to the 144th power. Now 144 times the logarithm of 2, (or -3010300) = 43-3483200, showing that the above mentioned denominator is a number of 44 digits, beginning 2230077 with 37 digits to follow. This inconceivably enormous num- ber represents the odds to 1 against the ob- served arrangement being the result of chance, even considering only one relation out of several mentioned above, all of which present the same order of antecedent improbability. Thus Laplace was led to his conception of a vast rotating nebulous disk, from the grad- ual contraction of which, and the consequent throwing off of rings, breaking up into globes, all revolving and rotating in one common di- rection and nearly in the same level, the solar system was formed. This hypothesis, how- ever, does not explain the distribution of the masses of the solar system ; one planet (Jupi- ter), for example, containing nearly $ of all the matter outside the sun, and Saturn and Jupiter together containing about % of all that matter. Accordingly, the present writer has suggested a modification of it, in which, starting from some such primary condition as that assumed by Herbert Spencer, the various parts of the solar system were formed by pro- cesses of aggregation such as are still going on (though now with extreme slowness). For the motions of the flocculi of Spencer (or of the parts, whatever their nature, from which the system was to be formed) would be more and 1 more rapid with proximity to the central aggre- gation, according to well known dynamical laws, Accordingly, subordinate aggregations would form with difficulty close by the sun; and hence we can understand the smallness of the