372 POPULAR LECTURES AND ADDRESSES. 



of the meteoric theory of the origin of the sun's 

 heat, may be accepted as having the highest degree 

 of scientific probability that can be assigned to any 

 assumption regarding actions of prehistoric times. 

 The essential principle of the explanation is this : 

 at some period of time, long past, the sun's initial 

 heat was generated by the collision of pieces 

 of matter gravitationally attracted together from 

 distant space to build up his present mass ; and 

 shrinkage due to cooling gives, through the work 

 done by the mutual gravitation of all parts of the 

 shrinking mass, the vast heat-storage capacity in 

 virtue of which the cooling has been, and continues 

 to be, so slow. 



In some otherwise excellent books it is 

 " paradoxically " stated that the sun is becomirrg 

 hotter because of the condensation. 1 Paradoxes 



1 [Note of February 21, 1887. The " paradox " referred to he.re, 

 is, as I now find, merely a misstatement (faulty and manifestly 

 paradoxical through the omission of an essential condition) of an 

 astonishing and most important conclusion of a paper by J. Homer 

 Lane, which appeared in the American Jmirnal of Science, for July, 

 1870 (referred to more particularly on p. 398 below). In Newcomb's 

 Popular Astronomy, first edition, p. 508, the omission is supplied in 

 a footnote, giving a clear popular explanation of the dynamics of 



