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THE POPULAR SCIENCE MONTHLY. 



prepared hiin for the broad investigation 

 which has led to the writing of the present j 

 volume ; but the problems of the nebular 

 hypothesis have long occupied a large 

 amount of attention with him, and been 

 made a subject of his college lectures, so 

 that he has made it a point to master the 

 various special questions that have recently 

 come forward in connection with this sub- 

 ject. We know of no other work in which 

 the reader can find a full, connected, and 

 systematic presentation of the results of 

 cosmical research that will compare with 

 this, and we are especially glad to see that 

 the publishers have put it at a reasonable 

 and popular price. 



No sufficient account of the contents of 

 the book can be offered in the space at our 

 command, but we give an imperfect outline 

 of the main features of the exposition. 



The book is divided into four parts, of 

 which Part I, entitled " World-Stuff," treats 

 of the process by which the constituent 

 particles of worlds become aggregated into 

 spheroidal masses. The meteoric matter 

 which is constantly falling upon the earth 

 in masses varying from dust-particles to 

 meteorites of several tons weight, the zo- 

 diacal light, which polariscopic study shows 

 to be reflected sunlight, comets, which are 

 now known to be simply conglomerations of 

 cosmical dust, the rings of Saturn, and the 

 irresolvable nebula?, all go to show that a 

 vast amount of matter such as our earth is 

 made of, must exist diffused in space. "All 

 the moving bodies of our system must be 

 continually pelted by these cosmical atoms, 

 and the aggregate result of these collisions 

 must, in thousands or millions of years, 

 affect their motions. Supposing the mo- 

 tions of the cosmical atoms to have no pre- 

 vailing direction, it is evident that the mo- 

 tions of the planets, satellites, and comets 

 of our system would cause them to meet 

 more of these atoms than the total number 

 which would overtake them. The result 

 would, therefore, be a resistance to the 

 movement of these bodies, and the effect of 

 this would be an acceleration of their mo- 

 tions and a shortening of their periods. I 

 venture the opinion that this cause is a 

 more efficient resistance than the supposed 

 ethereal medium." These material particles 

 are drawn by mutual attraction into groups, 



and any central attractive force, as of a sun 

 or planet, would also cause them to aggre- 

 gate, by deflecting their motions into con- 

 verging lines. But, in the presence of two 

 or more attractive centers, as in the present 

 constitution of the cosmos, it is impossible 

 that any mass shall fall directly upon its 

 center of attraction ; hence every body 

 would tend to circulate about every other 

 body. But the resulting movements would 

 be so infinitely complex as to precipitate 

 countless collisions of particles and masses. 

 Each group or swarm which gradually forms 

 will have a progressive motion along a path 

 having the essential character of an orbit 

 around some dominant center of attraction. 

 If any condition of interplanetary matter 

 exists in space, its resistance would cause 

 the smaller particles to fall behind, and the 

 whole swarm to assume an elongated fan- 

 shape. The attractions that control these 

 motions would be feeble; sometimes the 

 controlling one would be only that of an- 

 other cosmical swarm. Most of these swarms 

 of cosmical dust would simply float poised 

 in space, growing by accession of particles, 

 and occasionally coalescing with other clouds, 

 until an aggregation is formed large enough 

 to be called a nebula. From these various 

 attractions and collisions the nebula would 

 have acquired a rotary motion. It would 

 assume the form of an oblate spheroid, and, 

 as the cloud-like mass cooled, the conse- 

 quent contraction would increase the speed 

 of rotation, until an equatorial ringlet of 

 particles gained a centrifugal tendency equal 

 to the centripetal. Further contraction 

 would cause the main body of the spheroid 

 to shrink away from this ring, which would 

 then rotate independently. We might sup- 

 pose that successive slender ringlets would 

 become detached until the whole mass was 

 converted into an essentially continuous disk, 

 for the attraction of the ring first separated 

 would be added to the centrifugal force of 

 the circlet of particles nearest it, and so on. 

 But every successive addition to the annular 

 mass increases its distance from the next 

 ringlet of particles, and upon this its influ- 

 ence, though increasing with the growth of 

 the ring, diminishes as the square of the 

 distance increases. As a result, " an annu- 

 lar mass of relatively considerable amount 

 would separate, and a secular interval would 



