THEORIES OF EVOLUTION OF SOLAR SYSTEM 61 



90 mark. Tie a small weight to the middle brad, using a piece 

 of string more than 6" long. When the three brads are 

 horizontal, the string should cross the degree mark. 



b. Holding the instrument in the left hand, sight along the 

 three brads at some elevated object and read where the string 

 crosses the degree marks. This is the angular altitude from 

 that position. Approach the object and again sight. Why 

 is there a difference ? Has the object become taller ? 



c. " Sight " at the sun. To do this hold a piece of paper 

 near one end of the clinometer and obtain the three shadows 

 of the brads in the same place. The reading indicates the 

 altitude of the sun. Walk toward the sun and obtain its al- 

 titude. Is there any difference? Explain. Wait half an 

 hour and repeat, giving an explanation of the results. 



43. THEORIES OP THE EVOLUTION OF THE SOLAR SYSTEM 



There are two general theories which must not be accepted 

 as facts, but as theories. There is much which supports each 

 of these theories, and likewise there are many things which 

 are not explained by either of them. 



La Place formulated a Ring Hypothesis, which, stated 

 briefly, is as follows: In the beginning there was a large 

 mass of heated gaseous material which began slowly to revolve, 

 and as it cooled it contracted. As the contraction took place 

 the motion became faster and faster, until the centrifugal 

 force was greater than the gravitational force, so that a ring 

 around its equator was thrown off into space. Ring after 

 ring left the parent mass, each ring contracting into a sphere, 

 one of which is the earth. The other planets were formed in 

 the same way. There remained a central mass, which is our 

 present sun. 



