10 Introductory Chapter [CH. i 



definitely marked cluster in Perseus, and less noteworthy clusters in Scorpius- 

 Centaurus and Cygnus. There seems to be a tendency for clusters to assume 

 a flattened shape, the flattening in the case of the Ursa Major cluster being 

 almost complete*, so that the stars lie almost in a plane. 



7. ' We have now mentioned five different types of structure found in 

 the sky, each of which shews a more or less pronounced uniformity. The 

 aim of a scientific cosmogony must be to trace these and other uniformities 

 to their sources. When we find a formation repeated many times with only 

 slight variations, we may feel fairly confident that its origin is in every case 

 the same. The problem of cosmogony is to discover these origins and to 

 prove that they would lead to the observed formations. 



The various uniformities of structure are by no means of equal importance. 

 A purely objective view would perhaps regard the finding of the origins of 

 planetary systems as the least important problem of cosmogony, but, for 

 reasons which can readily be understood, cosmogony has always been more 

 concerned with this special problem than with any of the others. Indeed 

 until quite recent years not enough was known of the universe outside our 

 solar system for the problems of cosmogony to have assumed a definite shape 

 except in reference to our own system. We now proceed to give a short 

 account of some of the various theories of planetary origin which have been 

 propounded. 



THEORIES OF COSMOGONY 

 I. THE NEBULAR HYPOTHESIS OF KANT AND LAPLACE 



8. Of all theories of cosmogony, the most enduring, and infinitely the 

 most famous, has been the Nebular Hypothesis, commonly associated with the 

 names of Kant and Laplace. Kant's theory was first given in his Allgemeine 

 Naturgeschichte und Theorie des Himmels in 1755 ; Laplace published the 

 outlines of his theory in 1796 in his Exposition du Systeme du Monde, 

 developing his ideas further in later editions. Laplace seems to have been 

 quite unacquainted with the earlier speculations of Kant ; indeed he speci- 

 fically states that Buffon was, so far as he knew, the only philosopher who, 

 since the true nature of the solar system had been known, had speculated as 

 to the origin of the planets and their satellites. Thus we have two theories, 

 of distinct and independent origins, trying to explain the same phenomena. 

 Kant's theories, however, attempted to explain the whole stellar universe, 

 while Laplace limited himself to the solar system. 



* See a description by H. H. Turner, The Observatory 34 (1911), p. 246. 



