262 



SCIENCE. 



[K S. Vol. XXII. No. 557. 



been solved, much uncertainty remains as 

 to the extent to which our conclusions will 

 be applicable to actual celestial bodies. 



We begin, then, with a rotating liquid 

 planet like the earth, which is" the first 

 stable species of our family. We next im- 

 part in imagination more rotation to this 

 planet, and find by mathematical calcula- 

 tion that its power of resistance to any sort 

 of disturbance is less than it was. In 

 other words, its stability declines with in- 

 creased rotation, and at length we reach a 

 stage at which the stability just vanishes. 

 At this point the shape is a transitional 

 one, for it is the beginning of a new species 

 with different characteristics from the first, 

 and with a very feeble degree of stability 

 or power of persistence. As a still fur- 

 ther amount of rotation is imparted, the 

 stability of the new species increases to a 

 maximum and then declines until a new 

 transitional shape is reached and a new 

 species comes into existence. In this way 

 we pass from species to species with an 

 ever-increasing amount of rotation. 



The first or planetary species has a cir- 

 cular equator like the earth; the second 

 species has an oval equator, so that it is 

 something like an egg spinning on its side 

 on a table ; in the third species we find 

 that one of the two ends of the egg begins 

 to swell, and that the swelling gradually 

 becomes a well-marked protrusion or fila- 

 ment. Finally, the filamentous protrusion 

 becomes bulbous at its end, and is only 

 joined to the main mass of liquid by a 

 gTadually thinning neck. The neck at 

 length breaks, and we are left with two 

 separated masses which may be called 

 planet and satellite. 



In this ideal problem the successive 

 transmutations of species are brought 

 about by gradual additions to the amount 

 of rotation with which the mass of liquid 

 is endowed. It might seem as if this con- 

 tinuous addition to the amount of rotation 



were purely arbitrary and could have no 

 counterpart in nature. But. real bodies 

 cool and contract in cooling, and I must 

 ask you to believe that the effects of an ap- 

 parently arbitrary increase of rotation may 

 be produced by cooling. 



The figures which I succeeded in draw- 

 ing, by means of rigorous calculation, of 

 the later stages of this course of evolution, 

 are so curious 'as to remind one of some 

 such phenomenon as the protrusion of a 

 filament of protoplasm from a mass of liv- 

 ing matter, and I suggest that we may see 

 in this almost life-like process the counter- 

 part of at least one form of the birth of 

 double stars, planets and satellites. 



My Cambridge colleague, Jeans, has also 

 made an interesting contribution to the 

 subject by attacking the far more difficult 

 case where the rotating fluid is a compres- 

 sible gas. In this case also he finds a 

 family of types, but the conception of com- 

 pressibility introduced a new set of consid- 

 erations in the transitions from species to 

 species. The problem is, however, of such 

 difficulty that he had to rest content with 

 results which were rather qualitative than 

 strictly quantitative. 



It can not be doubted that the supposed 

 Laplacian sequence of events possesses a 

 considerable element of truth, yet these 

 latter schemes of transformation can be 

 followed in closer detail. It seems, then, 

 probable that both processes furnish us 

 with crude models of reality, and that in 

 some cases the first and in others the sec- 

 ond is the better representative. 



The moon's mass is one eightieth of that 

 of the earth, whereas the mass of Titan, 

 the largest satellite in the solar system, is 

 one forty-sixth hundredths of that of 

 Saturn. On the ground of this great dif- 

 ference between the relative magnitudes of 

 all other satellites and of the moon, it is 

 not unreasonable to suppose that the mode 

 of separation of the moon from the earth 



