THE DIRECTION OF MOTION. 235 



sole law; since, by the hypothesis, both forces are every- 

 where in action. Bnt very frequently the one kind of force 

 is so immensely in excess that the effect of the other kind 

 may be left out of consideration. Practically we may say 

 that a body falling to the Earth, follows the line of great- 

 est traction ; since, though the resistance of the air must, if 

 the body be irregular, cause some divergence from this line, 

 (quite perceptible with feathers and leaves,) yet ordinarily 

 the divergence is so slight that we may omit it. In the same 

 manner, though the course taken by the steam from an ex- 

 ploding boiler, differs somewhat from that which it would 

 take were gravitation out of the question ; yet, as gravitation 

 affects its course infinitesimally, we are justified in asserting 

 that the escaping steam follows the line of least resistance. 

 Motion then, we may say, always follows the line of great- 

 est traction, or the line of least resistance, or the resultant 

 of the two: bearing in mind that though the last is alone 

 strictly true, the others are in many cases sufficiently near 

 the truth for practical purposes. 



Movement set up in any direction is itself a cause of fur- 

 ther movement in that direction, since it is the embodiment 

 of a surplus force in that direction. This holds equally with 

 the transit of matter through space, the transit of matter 

 through matter, and the transit through matter of any kind 

 of vibration. In the case of matter moving through space, 

 this principle is expressed in the law of inertia — a law on 

 which the calculations of physical astronomy are wholly 

 based. In the case of matter moving through matter, we 

 trace the same truth under the familiar experience that any 

 breach made by one solid through another, or any chan- 

 nel formed by a fluid through a solid, becomes a 

 rout along which, other things equal, subsequent move- 

 ments of like nature take place. And in the case of mo- 

 tion passing through matter under the form of an im- 

 pulse communicated from part to part, the facts of mag- 

 netization go to show that the establishment of undulations 



