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CHAPTER IX. 



THE CIRCULAR MOTION OF FLUIDS. 



1. The hypothesis of the Cartesian theory. 



2. Newton s hypothesis as to the law of internal friction in fluids, the 



motion of a cylindrical vortex Bernoulli s objections to this re 

 sult. Prop. LI. 



3. Some difficulties of the Cartesian theory which are considered by 



Newton. 



a. The Sun s rotation. LII. Cor. 4. 

 /3. The third law of Kepler. Scholium LIU. 

 7. The two first laws of Kepler. Scholium LIII. 

 5. The density of the planets, LIU. 



e. The disturbance of the Sun s motion by the planetary vortices. 

 LII. Cor. 5, &c. 



1. BEFORE the time of Newton the Cartesian theory was 

 believed in by almost every nation of Europe. It was 

 therefore necessary that some notice should be taken of 

 these opinions in a work in which a totally different system 

 was proposed for the first time. Accordingly Newton has 

 devoted a section to the consideration of this hypothesis. 

 The philosophy of Descartes was naturally a very popular 

 one. Its explanations of the general facts of astronomy 

 were so exceedingly simple that it required no previous 

 learning to enable any one to understand them. Men 

 were pleased to think that in a few minutes they could 

 learn the cause of the motion of the planets. The sun was 

 supposed to be the centre of a vast vortex or whirlpool, the 

 density and angular velocity of the various parts of which 

 were different. The planets being placed, each in that 



