HA RD WICKE'S SCIENCE-GOSSIP. 



49 



THE NEBULAR HYPOTHESIS. 



No. 3. — Conclusion. 

 By E. P. RIDLEY 



L E have seen that 

 the oval form of 

 a nebula is due 

 to the combined 

 effects of gravita- 

 tion and rotatory 

 movement. But 

 this implies an 

 earlier state in 

 which the figure 

 was irregular. 

 Now while the 

 heavier portions 

 of the nebulous 

 mass were being 

 drawn together so 

 as to acquire a 

 spheroidal con- 

 tour, the lighter 

 portions, floating 

 further from the 

 centre of gravity, would remain like detached shreds of 

 cloud, or as long luminous streaks. And while most 

 of these — though probably not all — would ultimately 

 be compelled by gravitation to revolve about the 

 centre of the mass, the lightest and outermost shreds 

 would be a long time in acquiring a definite direction 

 of rotation, and some would not be drawn in until 

 the main mass had become considerably contracted. 

 Such late arrivals coming from a great distance and 

 therefore having small angular velocities, would move 

 in very eccentric ellipses, and while they would come 

 from all parts of the space which the mass originally 

 occupied, they would come chiefly from regions remote 

 from the plane in which integration had been most 

 marked— that is, they would come from the poles of 

 the nebula rather than from its equatorial regions — 

 and having failed to accompany the retreating mass 

 of the nebula while it was first acquiring a definite 

 direction of rotation, their own revolutions would be 

 determined chiefly by their irregular shapes, and would 



No. 303.— March 1890. 



as likely be retrograde as direct. All this is true of 

 comets — they come chiefly from high solar latitudes, 

 along immensely eccentric orbits and in directions 

 which are indifferently direct or retrograde. Many of 

 them indeed have parabolic and hyperbolic paths, 

 showing that they must be strangers to our system, 

 and have come from distant regions of space. But 

 as we can understand that some outliers of the nebulous 

 mass would, by escaping the power of gravitation, get 

 away into space, so outliers escaping from other 

 systems, may find their way into ours. Comets are 

 nebulous in constitution and of inconsiderable mass, 

 too small to produce planets or even asteroids, but 

 they probably concentrate into streams of meteors or 

 shooting stars, a 100,000,000 of which, it is computed, 

 are poured down upon the earth and planets each 

 year. The connection between meteor streams and 

 comets has been firmly established, and the chemical 

 constitution of comets and meteors does not differ 

 much from the chemical constitution of our earth ; 

 and though differing in their physical characteristics- 

 from ordinary rock, Sir Henry Roscoe says the earth's 

 interior mass may partake of the physical nature of 

 these metr.llic meteorites, and that the existence of 

 such interior masses of metallic iron may go far to ex- 

 plain the well-known magnetic condition of our planet. 



We have now considered a large number and a 

 great variety of phenomena which arise in connection 

 with the solar system — I will not stop to recapitulate 

 them, — and with one solitary exception the nebular 

 hypothesis explains all. Taken separately, many of 

 the phenomena are alone almost sufficient, but 

 together their influence is irresistible. I shall pro- 

 ceed now briefly to consider the future of the 

 system, and then to notice two other theories, one 

 accounting for the formation and heat of the original 

 nebula, and the other a rival theory by Mr. Proctor. 



The moon appears to afford an example of the 

 universal death which, in a remote future, awaits all 

 the members of the solar system. Indeed, this 

 appears to be a necessary corollary due to the con- 



D 



