58 The Outline of Science 



Moreover, there are great difficulties in Laplace's theory of the 

 separation of .successive rings from the main mass, and of the 

 condensation of a whirling gaseous ring into a planet. 



So it has come about that the picture of a hot gaseous nebula 

 revolving as a unit body has given place to other pictures. Thus 

 Sir Norman Lockyer pointed out (1890) that the earth is gather- 

 ing to itself millions of meteorites every day; this has been going 

 on for millions of years; in distant ages the accretion may have 

 been vastly more rapid and voluminous; and so the earth has 

 grown! Now the meteoritic contributions are undoubted, but 

 they require a centre to attract them, and the difficulty is to ac- 

 count for the beginning of a collecting centre or planetary 

 nucleus. Moreover, meteorites are sporadic and erratic, scat- 

 tered hither and thither rather than collecting into unit-bodies. 

 As Professor Chamberlin says, "meteorites have rather the 

 characteristics of the wreckage of some earlier organisation than 

 of the parentage of our planetary system." Several other 

 theories have been propounded to account for the origin of the 

 earth, but the one that has found most favour in the eyes of 

 authorities is that of Chamberlin and Moulton. According to 

 this theory a great nebular mass condensed to form the sun, from 

 which under the attraction of passing stars planet after planet, 

 the earth included, was heaved off in the form of knotted spiral 

 nebula?, like many of those now observed in the heavens. 



Of great importance were the "knots," for they served as 

 collecting centres drawing flying matter into their clutches. 

 Winterer part of the primitive bolt escaped and scattered was 

 drawn out into independent orbits round the sun, forming the 

 "planetesimals" which behave like minute planets. These plane- 

 tr si ii m Is formed the food on which the knots subsequently fed. 



The Growth of the Earth 



It has been calculated that the newborn earth the "earth- 

 knot" of Chamberlin's theory had a diameter of about 5,500 



