162 NEWTON S PEINCIPIA. 



II. 



HITHERTO we have considered all motion as performed 

 in vacuo, or in a medium which offers no resistance to the 

 action of forces upon bodies moving in any direction. It 

 was necessary that the subject should first be discussed 

 upon this supposition ; and the hypothesis agrees with 

 the fact as far as the motions of the heavenly bodies are 

 concerned. But all the motion of which we have any 

 experience upon or near the surface of the earth, is per 

 formed in the atmosphere that surrounds our globe ; and 

 therefore, as regards all such motion, a material allow 

 ance must be made for the resistance of the air when we 

 apply to practice our deductions from the theory. It is 

 also obvious that a still greater effect will be produced 

 upon moving bodies, if their motion is performed in a 

 denser fluid, as water. Further, the pressure and motion 

 of fluids themselves form important subjects of considera 

 tion, independent of any motion of bodies through them 

 and impeded by them. These several matters form the 

 subject of the sciences of Hydrostatics, Hydraulics, and 

 Pneumatics; the first treating of the weight and pressure 

 of watery fluids, the second of their motion, the third of 

 aeriform or elastic fluids. They are discussed in the 

 Second Book of the Principia. It consists of Nine Sec 

 tions ; of which the First Three treat of the motion of 

 bodies to which there is a resistance in different propor 

 tions to the velocity of the motion ; the Fourth treats of 

 circular or rather spiral motion in resisting media; the 

 Sixth, of the motion and resistance of pendulums ; and 

 part of the Seventh discusses the motion of projectiles ; 

 while the rest of the Seventh, and the whole of the four 



