188 SCIENCE IN GRECO-ROMAN ANTIQUITY 



Further, according as the movement is natural or 

 violent, the resistance and the force manifest themselves 

 differently. In the natural movement, the force is 

 constituted by the quality of heaviness or lightness 

 which impels a body towards its position of equilibrium 

 and which acts inexhaustibly until this point is reached 

 by the body. As to the resistance, it is simply that 

 offered by the medium traversed, for instance the air 

 in the fall of a heavy body. 



Observation shows us, besides, that the natural 

 movement, in as far as it is rectilinear, is accelerated 

 {Simplicius in Aristotelis. Diels, Bk. V, ch. vi, p. 916). 



When a streamlet of water falls from a height, from 

 a gutter, for example, it appears continuous near its 

 origin, but soon the acceleration of the fall detaches 

 the drops of water from one another and they fall to 

 the ground separately. 



When a stone falls from a height, it strikes an 

 obstacle more violently if it is stopped towards the 

 end of its fall than at the middle or beginning ; this 

 more violent impact is the sign of a greater velocity. 1 

 Moreover, the theory confirms the observation. The 

 rectilinear movement cannot go on for ever, it has a 

 beginning and an end. Hence, starting from rest at 

 a determinate moment of the duration, a moving body 

 only passes from a zero velocity to a given velocity 

 by means of an acceleration, and this acceleration 

 continues for the same reasons as it began. It only 

 ends when the moving body has reached its goal, its 

 position of equilibrium. 2 



In violent movements such as the traction of a 

 cart and the towing of a vessel, resistance is represented 

 by the weight of the object to be moved, and force by 

 the motive power continuously acting on this object. 

 The movement of a projectile in -the air is a special 



1 13 Duhem, Systeme, I, p. 388. 



2 24 Sageret, Systeme, p. 214. 



