28 FOBCE AND THE LAWS OF MOTION 



have started the motion, so that we get an approximation to uniform 

 motion in a straight line. The forces acting to alter this motion are 

 (a) the resistance of the air and (&) the weight of the bullet. The former, 

 as we have seen, tends to stop the motion by pressure on the ends and sides 

 of the bullet ; the latter tends to drag the bullet down to the earth, and 

 so causes it, instead of describing a straight line, to describe a path which 

 curves downward towards the earth. 



19. The conception of uniform motion in a straight line, or of 

 rest (the particular case of uniform motion in which the velocity 

 is nil), as being the normal state of a body is due to Galileo (1564- 

 1642). An interesting account of the discovery of this law will be 

 found in Chapter II of Mach's Science of Mechanics? or in Chap- 

 ter IX of Cox's Mechanics? 1 Before the time of Galileo it was 

 commonly supposed, on the authority of Aristotle, that every body 

 had a natural place, and that its normal state was one of rest in 

 this natural place. For instance, a stone was supposed to sink in 

 water, not because the force of gravity was acting on it and setting 

 it into downward motion, but because its natural place was at the 

 bottom of the water; a cork was supposed to rise because its 

 natural place was at the top. Thus Girard, 3 in 1634, speaks of 

 " millions de matieres, qui sont disposees chacunes en leurs lieux," 

 and defines gravity as " la force qu'une matiere demonstre a son 

 obstacle, pour retourner en son lieu." Thus the effect of force, 

 before Galileo, was supposed to be to keep a body out of its 

 natural place. Galileo perceived that bodies had no natural 

 places at all, but natural states, namely of rest or of uniform 

 motion in a straight line, and the effect of force is not to move a 

 body from its natural place but to disturb it from its natural 

 state, i.e. to alter its speed. This discovery of Galileo is what is 

 expressed by Newton's first law of motion. 



20. Having settled what is meant by the natural state of a 

 body and also what is meant by force, namely that which tends 



1 Ernest Mach, Science of Mechanics (Eng. trans, by McCormack). 



2 J. Cox, Mechanics, Cambridge, University Press, 1904. 



3 In the Elzevir edition of Stevin, Leyden, 1634. See Cox, Mechanics, loc. cit. J 

 ante. 



