Page:Popular Science Monthly Volume 39.djvu/361

Rh variable stars when near their maximum brilliancy, and these may very possibly be due to the heat produced by meteoric collisions.

Prof. G. H. Darwin, the eminent Cambridge professor, has proved mathematically one point in favor of Lockyer's hypothesis, He shows that the conception of fluid pressure required by Laplace's nebular hypothesis, and which is applicable to a gas, is also applicable to a swarm of meteorites. The pressure exerted by a gas against the surface of an inclosing vessel is supposed to be the result of collisions between the component molecules of the gas, and Prof. Darwin shows that, if we supposed these molecules magnified up to the size of meteorites, their collisions will still give a quasi-fluid pressure, and that the law of gases will be applicable to a swarm of meteorites. One objection may be raised to this view, namely, that the ultimate molecules of a gas are supposed to be perfectly, or at least highly, elastic, while meteoric stones have very little elasticity. Prof. Darwin, however, points out that, when the meteorites come into collision, the heat generated by the shock volatilizes a portion of each, so that the result will be like that of an explosive, and consequently there will be nearly perfect elasticity. He finds, further, that the analogy with the theory of gases will hold good for the meteoric swarms from which the solar system—on Lockyer's hypothesis—is supposed to have been evolved, a swarm extending beyond the orbit of Neptune. He also finds that the swarm when widely diffused will be subject to gaseous viscosity, and will first rotate as a solid body, but when more contracted "the central portion will revolve more rapidly than the outside."

With reference to the origin of comets, Mr. Monck inclines to the opinion that some comets, at least, originated "within the limits of the solar system," and to this class he is disposed to assign "the four comets which have been connected with meteor swarms." He argues that some meteors may be of terrestrial origin, and suggests that possibly Lockyer's experiments may have been made with some of these terrestrial meteorites.

There seems to be another weak point in Prof. Lockyer's hypothesis, and that is that it offers no explanation of how the plan' ets and satellites of the solar system were evolved. This has been pointed out by Mr. Monck. He says: "Will any advocate of the meteoric theory give us an explanation of why all the planets and asteroids and the great majority of the satellites revolve in the same direction, why the orbits of the larger bodies of the system deviate so little from the circle, and why they are so nearly in the same plane? Till this is done I think the nebular hypothesis has in this case the advantage." A violent grazing collision between two dense meteoric swarms might, however, conceivably, be