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and moon were originally one fluid mass, that oscillations set up in the mass 

 by the tidal effects of the sun resulted in the separation of the mass into two 

 parts, that the two parts raised tides each in the other and that the friction 

 of these large tidal waves resulted in the separation of the two bodies to their 

 present distance and the lengthening of their rotation periods to their present 

 values. 



It is, no doubt, true that tidal friction does tend to lengthen the period 

 of rotation of the earth, and, if the fundamentals of mechanics are to be 

 trusted, this effect must result in an increased distance between the two 

 bodies. Some observational data in support of this theory appears in the 

 fact that the period of revolution of the moon about the earth coincides with 

 its period of rotation, and that probably the two planets nearest the sun 

 keep the same face to the sun. On the other hand we know that tidal fric- 

 tion or any other force has failed to change the length of our day by one-tenth 

 of a second in five thousand years. There has more recently come into gen- 

 eral favor another and a totally different theory, from Professors Chamber- 

 lain and Moulton, of the Departments of Geology and Astronomy, of Chicago 

 University. 



They suppose that the solar system took its form from a nebula, but from 

 a spiral and not from a spherical or spherodial nebula. Observationally 

 this supposition is sound. There is not in the sky, as far as I know, a nebula 

 of the sort assumed by La Place. There are thousands, perhaps hundreds of 

 thousands of the spiral sort. Of all the nebulae that have any regular shape 

 the spirals outnumber all others. There are a few so called planetary nebulae 

 which in the telescope look spherical, but which in a long exposure photo- 

 graph show some other form. Some of them may be hollow spheres, but 

 none appears as La Place's nebula was supposed to be. There are a few in 

 the form of a ring with a star at the center, but again it must be remarked 

 that this form in not the required form. 



In a spiral nebula the matter forming the arms of the spiral is usually the 

 smaller part of the whole mass, the greater part being at or near the center. 

 If the law of gravitation holds among them, and we have never found an 

 exception to it, then the particles in the arms of the spiral must be in motion 

 in elliptical orbits about the central mass, the parts nearer the center moving 

 faster than the more remote parts. This means that the arms must with 

 time become more closely wrapped about the central mass and that any one 



