ORIGIN AND DEVELOPMENT OF THE EARTH 309 



is a beautifully adjusted and harmonious family of planets 

 which we may be sure has not been seriously disturbed during 

 many millions of years in the past. 



THEORIES OF ORIGIN 



The heavenly bodies have always interested men to such 

 a degree that many attempts have been made to explain their 

 origin and arrangement. Up to the sixteenth century it was 

 believed that the earth was the central and largest part of the 

 universe, and that the sun, moon, and stars moved about it. 

 Men were very generally of the opinion that these bodies 

 were created in their present state and were maintained for 

 the express purpose of giving light to the human inhabitants 

 of the earth. It was only after Copernicus, in the sixteenth 

 century, showed that the earth is merely a small member of 

 a great system, that this narrow and egotistical view began to 

 give way to a broader conception of the heavens. 



The nebular theory of Laplace. Near the end of the 

 eighteenth century the distinguished French mathematician 

 Laplace worked out a most ingenious theory which seemed 

 for a time to account for nearly every peculiarity of the solar 

 system. He suggested that it was originally derived from a 

 huge spheroidal mass of gas, which was so hot that even the 

 metals and the materials which form the rocks of to-day were 

 then expanded into an incandescent vapor many times thin- 

 ner than air. As this nebula gradually cooled, it shrank, 

 and its shrinkage made it rotate faster upon its axis. The 

 faster it whirled the stronger became the centrifugal force on 

 the outside of the spheroid. This tendency of things to fly 

 off into space was at first counteracted by the attraction of 

 gravity within ; but eventually it became equal to gravity on 

 the equator of the spheroid and the material there then ceased 

 to contract. 



The equatorial portion was then left as a ring encircling 

 the ever shrinking remainder. This ring is supposed to have 



