292 POPULAR SCIENCE MONTHLY. 



It would be interesting to recall the strange traditions in which 

 j)rimitive peoples have recorded their vague imaginings of the origin of 

 things. But the absence of even an attempt at careful reasoning 

 renders such tales of no value for our present purpose. The Greek 

 philosophers were not oblivious to the value of observation as a check 

 on speculation regarding the solar system, but the instruments then 

 available were too crude to give accurate positions of the heavenly 

 bodies. Even Copernicus, though he established the sun at the center 

 of our system, and thus paved the way for the nebular hypothesis, re- 

 tained the epicycles of the Greeks. Kepler, basing his investigations 

 upon the observations of Tycho Brahe, proved that the planets move 

 in ellipses with the sun at the focus, and removed all vestige of doubt 

 as to tbe general plan of the solar system. The harmony which char- 

 acterizes the motions of the planets and a knowledge of the effect of 

 gravitation le.d Kant to formulate an explanation of the origin of the 

 solar system, which subsequently found more perfect expression in the 

 Jiebular hypothesis of Laplace. 



In this hypothesis Laplace seeks to account for the formation of 

 the sun and planets through the contraction of a vast nebulous cloud, 

 wliich once filled the entire solar system, extending to the orbit of Nep- 

 tune. This mass, which he considered to be fiery hot, was supposed to 

 be in rotation. As it cooled, through radiation into space, it contracted 

 toward the center. The result of this contraction was to increase the 

 velocity of rotation, and when through increasing velocity the centrif- 

 ugal force at the periphery counterbalanced the attraction of the cen- 

 tral mass, a ring Avas thrown off. Further contraction resulted in the 

 formation of other rings, in each of which the matter collected about 

 its densest part, and thus produced a planet. Before they had time to 

 cool these planets in turn threw off rings, which, with the single excep- 

 tion of Saturn's ring system, condensed into satellites. 



This celebrated hypothesis, though unsupported by mathematical 

 proof, has occupied a dominant position since the time of its publica- 

 tion more than a century ago. It has been subjected to much criticism, 

 but most of the objections raised by Faye and others have been met by 

 modifications of the hypothesis. Of late it has encountered fresh 

 attacks on the part of Chamberlin and Moulton, and it now seems 

 doubtful whether it will be possible to overcome their criticisms, which 

 are based on dynamical considerations. It may prove to be sufficient, 

 however, to forsake the lenticular mass of vapor predicated by Laplace 

 in favor of the spiral form which Keelor has shown to characterize so 

 many nebula.'. 



The nebular hypothesis seeks to account for a system like our own, 

 v,"herein a central sun is surrounded by planets and satellites, originally 

 self-luminous, but ultimately cooled to the point where they are lumi- 



