THE METEORITIC HYPOTHESIS. 347 



variable stars when near their maximum brilliancy, and these 

 may very possibly be due to the heat produced by meteoric col- 

 lisions. 



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 La- 

 place'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-Huid pressure, and that the law of gases will be ap- 

 plicable to a swarm of meteorites. One objection may be raised 

 to this view, namely, that the ultimate molecules of a gas are sup- 

 posed 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 gener- 

 ated 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 Nep- 

 tune. 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 lim- 

 its 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 hy- 

 pothesis, and that is that it offers no explanation o J f 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 sup- 



