The Earth and its Early History 



Under these circumstances he supposed that the gas 

 settled down under the law of gravitation into a vast 

 sphere, much denser towards its centre than on its 

 surface, and that this sphere was slowly revolving in the 

 same direction as the planets now move round the Sun. 

 The circumference of the sphere was greater than the 

 orbit of Neptune, which has a radius of 2800 millions of 

 miles. Under the influence of its rotation the sphere 

 became slightly flattened at its poles and developed a 

 slight bulge around its equator. 



The sphere of gas was slowly losing its heat into 

 surrounding space, and this brought about a fall of 

 temperature and a gradual shrinking, so that it grew 

 slowly smaller and smaller, and as a consequence it 

 rotated faster and faster. 



, This can be illustrated by tying a small weight or 

 stone to one end of a string, attaching the other end to 

 a stick, and then whirling it round so as to allow the 

 string to wrap round the stick, thus shortening the radius 

 of swing of the stone. It will be seen that as the free 

 portion of the string becomes shorter the stone moves 

 faster. 



If we could whirl the stone sufficiently quickly the 

 string would break and the stone would fly off, because 

 the centrifugal force had become greater than the tensile 

 strength of the string. 



In the great revolving nebula the force of gravity 

 took the place of the string, and Laplace thought that as 

 the sphere shrank and the motion became faster, a stage 

 was reached when the centrifugal force in the bulge 

 round the equator of the sphere became equal to the 

 force of gravity. 



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