PLANETESIMAL THEORIES OF THE EARTH'S ORIGIN. 187 



To learn continually more and more of His thoughts, as 

 revealed in His works, is the highest reward of the student of 

 nature ; and increased powers of vision, whether with the 

 telescope or the microscope, open ever-widening fields of know- 

 ledge and new problems to be solved. In every direction the 

 search for truth reaches no limit; and in the themes of this 

 paper, although much has been ascertained, infinitely more 

 remains for inquiry. 



The nebular hypothesis or theory may well be called the 

 grandest generalization in all the range of the natural sciences. 

 As most elaborately stated by the eminent astronomer and 

 mathematician, Laplace, in his Exposition die Systcmc du Monde, 

 this theory traces the beginning and development of the solar 

 system from an original gaseous nebula, an exceedingly 

 tenuous and intensely heated cloud of matter, extending in 

 & spheroidal form beyond the orbit of Xeptune, the outer- 

 most planet. By its gravitation and resulting contraction, the 

 nebula is supposed to have acquired a movement of rotation, 

 with polar flattening. Whenever the outer equatorial belt of 

 the revolving nebula attained a centrifugal force exceeding 

 the attraction toward the central mass, a part would be left 

 behind, either as a relatively small revolving nebulous body, 

 or as a ring of such matter, somewhat like the rings of 

 Saturn. Later the ring, if it was at first of that form, would 

 be broken ; and, finally, the detached mass would be gathered 

 into a globe, which, in its condensation, would form satellites 

 in the same manner as outer parts of the great central mass 

 formed the successive planets. 



Under this theory the principal features of our planetary 

 system, implying unity of origin and development, find a con- 

 sistent general explanation. Professor Charles A. Young has 

 •enumerated these features, which could only have originated by 

 some long process of orderly evolution, as follows : — * 



1. The orbits of the planets are all nearly circular. 

 .2. They are all nearly in one plane, excepting considerable 

 divergence of some of the little asteroids. 



3. The revolution of all is in the same direction. 



4. There is a curiously regular progression of distances between 



the planetary orbits. 



5. There is a roughly regular progression of density, increas- 



ing both ways from Saturn. 



* Text-Bool of General Astronomy, 1893, p. 515. 



