298 SIMPLE AND COMPOUND EVOLUTION. 



motion is rapidly received; then, other forces will cause in 

 the aggregate appreciable modifications. Along with the 

 change constituting integration, there will take place sup- 

 plementary changes. The Evolution, instead of being sim- 

 ple, will be compound. 



The several propositions thus briefly enunciated require 

 some explanation. 



99. So long as a body moves freely through space, 

 every force that acts on it produces an equivalent in the 

 shape of some change in its motion. No matter how high 

 its velocity, the slightest lateral traction or resistance causes 

 it to deviate from its line of movement causes it to move 

 towards the new source of traction or away from the new 

 source of resistance, just as much as it would do had it no 

 other motion. And the effect of the perturbing influence 

 goes on accumulating in the ratio of the squares of the times 

 during which its action continues uniform. This same body, 

 however, will, if it is united in certain ways with other 

 bodies, cease to be moveable by small incident forces. 

 When it is held fast by gravitation or cohesion, these small 

 incident forces, instead of giving it some relative motion 

 through space, are otherwise dissipated. 



What here holds of masses, holds, in a qualified way, of 

 the sensible parts of masses, and of molecules. As the 

 sensible parts of a mass, and the molecules of a mass, are, 

 by virtue of their aggregation, not perfectly free, it is not 

 true of each of them, as of a body moving through space, 

 that every incident force produces an equivalent change of 

 position: part of the force goes in working other changes. 

 But in proportion as the parts or the molecules are feebly 

 bound together, incident forces effect marked re-arrange- 

 ments among them. At the one extreme, where the in- 

 tegration is so slight that the parts, sensible or insensible, 

 are almost independent, they are almost completely amen- 

 able to every additional action; and along with the con- 



