86 SCIENCE PROGRESS 



sufficient speed to detach the masses," when it extended to the 

 orbits of the several planets, as Laplace supposed. The evolution 

 of the planets by separation of rings of matter from the central 

 condensation, through rotational instability, must therefore be 

 abandoned. It is, however, possible that secondary condensa- 

 tion nuclei might be formed by gravitational instability within 

 the gaseous nebula; and this has been pointed out by Mr. Jeans 

 in papers which he has contributed to the Philosophical Trans- 

 actions of the Royal Society. 



We turn now to the alternative hypothesis developed by 

 the work of Prof. T. J. J. See. He has recently pointed out 

 that some remarkable anticipations of his views as to the action 

 of a resisting medium were made by Euler in 1749. The essen- 

 tial features of this hypothesis are that the Solar System has 

 been formed from a spiral nebula, and that the planets have 

 not been detached from the central mass through its rotation 

 but have been captured or added on from the outer parts of 

 the nebula. The roundness of the orbits of the planets and 

 satellites in general is due to the action of a resisting medium 

 which has reduced the size of their paths, and well nigh 

 obliterated the deviations from circularity. Just as the planets 

 have been captured by the sun's action, so in like manner 

 the satellites have been captured by their several primaries, not 

 detached by rotation of these latter. The moon too was 

 originally a planet, which neared the earth and was finally 

 captured and made a satellite. The asteroids or minor planets 

 between the orbits of Mars and Jupiter are the surviving 

 remains of millions of small planets, most of which have been 

 swallowed up by colliding with larger ones, though many are 

 still moving in independent paths round the sun. Our own 

 earth frequently encounters some of these objects, and we have 

 then a more or less brilliant " meteor shower." The satellites 

 having been captured in this way, it is not surprising that a few 

 of them should revolve in the opposite direction to the rest. 

 It is also remarkable that the paths of Phcebe (Saturn's 9th 

 satellite) and of the 8th satellite of Jupiter are much more oval 

 than those of any other known satellites, from which it would 

 appear that the density of the resisting medium must have been 

 very slight at the great distances from the planets at which they 

 revolve. The planetary rotations have also been produced by 

 the capture and absorption of small bodies ; and thus the larger 



