Social movements of these various orders severally conform to the twoderivative principles named at the outset. In the first place we see that,once set up in given directions, such movements, like all others, tend toproduce continuance in these directions. A commercial mania or panic, a currentof commodities, a social custom, a political agitation, or a popular delusion,maintains its course long after its original cause has ceased, and requiresantagonistic forces to arrest it. In the second place it is to be noted thatin proportion to the complexity of social forces is the tortuousness of socialmovements. The involved series of various processes through which a man isreturned to Parliament, or through which afterwards, by an Act he finallygets passed, certain doings of his fellow-citizens are changed, show this. §81. And now of the general truth above set forth what is our ultimateevidence? Must we accept it simply as an empirical generalization? or mayit be established as a corollary from a still deeper truth? The reader willanticipate the answer.
Suppose several tractive forces, variously directed, to be acting on agiven body. By what is known as the composition of forces, there may be foundfor any two of these, a single force of such amount and direction as to produceon the body an exactly equal effect. Such a resultant force, as it is called,may be found for any pair of forces throughout the group. Similarly, forany pair of resultants a single resultant may be found. And by repeatingthis course, all of them may be reduced to two. If these two are equal andopposite -- that is, if there is no line of greatest traction, motion doesnot arise. If they are opposite but not equal, motion arises in the directionof the greater. If they are neither equal nor opposite, motion arises inthe direction of their resultant. For in either of these cases there is anunantagonized force in one direction. And this residuary force must movethe body in the direction in which it is acting. To assert the contrary isto assert that a force can be expended without effect; and this involvesa denial of the persistence of force. If in place of tractions we take resistances,the argument equally holds; and it holds also where both tractions and resistancesare concerned. Thus the law that motion follows the line of greatest traction,or the line of least resistance, or the resultant of the two, is a necessarydeduction from that primordial truth which transcends proof.
