108 OUTLINES OF NATURAL PHILOSOPHY. 



178. The absolute elasticity of a spring is near- 

 ly proportional to its distance from its state of qui- 

 escence, or that at which its elasticity does not act ; 

 but the power of the same spring to impel a body, 

 must depend not only on its absolute force, but 

 also on the rate at which it overtakes the body. 



a. An arrow, for example, is impelled by the string of 

 a bow, not simply with a force proportional to the 

 distance of the string from the position in which it was 

 at rest, (to which the absolute elasticity of the bow is 

 probably proportional) but by a force which is less, 

 the nearer the velocity of the string and of the arrow 

 approach to an equality. 



This subject, however, has not been sufficiently exa- 

 mined. 



b. A spring is sometimes made to act with uniform force 

 by lengthening the lever to which it is applied in 

 the same proportion as its tension becomes less, or 

 as its distance from its state of quiescence dimi- 

 nishes. This is done in the case of a watch-spring. 

 The velocity acquired by the body impelled, is hers 

 so small that it may be neglected. 



179. When a moving body, whether splid or 

 fluid, acts upon another, the motion communi- 

 cated is less, the less the difference of their velo- 

 cities. 



2 The 



