PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 675 



The velocity of Mercury is nine times greater than that of Nep- 

 tune, — five times greater than* that of Saturn; three and a half 

 times greater than that of Jupiter; and twice as great as that of 

 Mars. At the distance of nine millions of miles from the sun, the 

 velocity of a body would be twice as great as that of Mercury; 

 and at the distance of four millions it would be four times as 

 great, or more than four hundred thousand miles an hour. It is 

 plain, therefore, that the resisting medium would produce its 

 greatest efiects between the sun and the orbit of Jupiter; and that 

 its effects would be gradually less as the orbital velocities de- 

 creased with the distances from the sun. 



To form a correct idea of the actual effects of the resisting 

 medium, upon the form and proportions of the embryo solar sys- 

 tem, or nebulous wheel, — we must consider what would necessa- 

 rily have been the form of the nebula, if the resisting medium had 

 not produced any effect upon its proportions. 



It is well known to mechanical philosophers that a fluid mass, 

 rotating rapidly on its axis, must necessarily tend to assume the 

 form of an oblate spheroid, — a double convex lens — a wheel, 

 thickest in the centre, and gradually thinner to the outer edge. 

 This would have been the form of the embryo solar system, had 

 it not been for the resisting etherial medium, and the greater 

 orbital velocities of the secondary matter near the sun. In the 

 very place (between the sun and Jupiter) where the planetary 

 matter would otlierwise have been most abundant, the resisting 

 medium caused it to be the least in quantity. The nebulous wheel, 

 instead of being thickest at the centre and gradually thinner from 

 the centre outwards, was, by the resisting medium, made thickest 

 at the centre and at the orbit of Jupiter, and very thin between 

 the sun and Jupiter, where the four small, dense planets are now 

 situated. 



Cause of the Relative Densities of the Planets. 

 Let us consider the effect of the resisting medium, combined 

 with the differences of orbital velocities upon the relative densi- 

 ties of the planets. The matter comprising the embryo solar sys- 

 tem was undoubtedly possessed of different degrees of density'. 

 The natural tendency of the condensing nebulous matter would 

 be to form itself into a countless number of meteoric or comet- 

 like masses — each separate mass consisting of a dense central 

 nucleus, and a less dense atmospheric envelope. Such, in fact, is 



