1865.] Phillips on the Planet Mars. 371 



tation that by reason of the " disproportionately large distance " 

 between the fourth and fifth satellites, a sixth might be expected 

 there. Bode's law, dating from 1772, does, indeed, place in strong 

 light the simple numerical proportions which prevail among the 

 planetary spaces ; and Olbers's discovery of the first of the small 

 planets which occupy the interval between Mars and Jupiter, and 

 thus confirm the law, adds much to the analogies which are the usual 

 basis for the conviction now so general of the common origin of all 

 parts of the solar system. What that common origin was has been 

 sketched for us on quite opposite plans by two eminent Frenchmen, 

 Buffon and Laplace. 



In the mind of the Count de Buffon, such a speculation assumed 

 a singular form. The matter of the planets was projected from the 

 sun by the shock of a comet. It was quite fluid, and gathered by 

 attraction, into globes rotating on their axes. In some cases, the 

 rapid rotation of planets while still fluid, caused parts to separate 

 and to collect into smaller globes, which circulated round the planets, 

 as these revolved round the sun. During the cooling, which Buffon 

 seems to have thought was pretty quick, the solid, liquid, and gaseous 

 matters were separated, and took their places according to gravity. 

 Most of the leading harmonies of the solar system are left unexplained, 

 others contradicted by this hypothesis. 



Laplace, who has honoured the scheme of Buffon by a notice of 

 its inherent defects, attempted to rise to the true theory on sounder 

 principles. He proposed what is now called the Nebular Hypothesis, 

 on the fundamental idea of an original expansion by heat of all the 

 substance of the planets into a vast atmosphere round the sun. 

 Bevolving round the sun, and subject to continual contraction by 

 cooling, the external sheaths of this atmosphere would be left behind 

 to continue their acquired movements, and finally to collapse into 

 globes, rotating and revolving according to the one original direction. 

 In a general sense, we may say the main facts of the solar system are 

 not opposed to this speculation. The small excentricity of planetary 

 orbits ; their common direction of motion ; the greater velocity of the 

 inner planets ; the various directions and great excentricity of comets ; 

 the zodiacal light ; the nebulae, which actually exist in a variety of 

 forms ; and, finally, the agreement of all the planets in their borrowed 

 light, their spheroidal figures acquired by revolution, the presence of 

 atmospheres, and the limited range of their densities — the larger 

 planets being, in general, least dense — all these circumstances are 

 apparently in harmony with this, the only general speculation at all 

 supported by mathematicians and astronomers. To change it into an 

 established theory by comparing it strictly with special phenomena of 

 the solar system, such as the inclinations of the axes of planets, the 

 order of their densities, the velocity of their rotations, the pecu- 

 liarities of their satellites, has been hardly attempted, except in 

 respect of the equality of the mean motions of rotation and revolu- 

 tion of the moon and other satellites, by Professor Haughton.* This 



* 'Trans. R. I. Academy.' 



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