prepared him for the broad investigation which has led to the writing of the present 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 subject. 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 zodiacal 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 motions of the cosmical atoms to have no prevailing direction, it is evident that the motions 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 motions 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 aggregate, by deflecting their motions into converging 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 another 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 consequent 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 suppose 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 influence, though increasing with the growth of the ring, diminishes as the square of the distance increases. As a result, "an annular mass of relatively considerable amount would separate, and a secular interval would