THE DIRECTION OF MOTION. 257 



parative greatness of forces is thus determined, no multipli- 

 cation of instances can add certainty to a law of direction of 

 movement which follows immediately from the persistence 

 of force. 



From this same primordial truth, too, may be deduced 

 the principle that motion once set up along any line, be- 

 comes itself a cause of subsequent motion along that line. 

 The mechanical axiom that, if left to itself, matter moving 

 in any direction will continue in that direction with undi- 

 minished velocity, is but an indirect assertion of the persist- 

 ence of force; since it is an assertion that the force mani- 

 fested in the transfer of. a body along a certain length of a 

 certain line in a certain time, cannot disappear without pro- 

 ducing some equal manifestation a manifestation which, 

 in the absence of conflicting forces, must be a further trans- 

 fer in the same direction at the same velocity. In 

 the case of matter traversing matter the like inference is 

 necessitated. Here indeed the actions are much more com- 

 plicated. A liquid that follows a certain channel through 

 or over a solid, as water along the Earth's surface, loses part 

 of its motion in the shape of heat, through friction and col- 

 lision with the matters forming its bed. A further amount 

 of its motion may be absorbed in overcoming forces which 

 it liberates; as when it loosens a mass which falls into, and 

 blocks up, its channel. But after these deductions by trans- 

 formation into other modes of force, any further deduction 

 from the motion of the water is at the expense of a reaction 

 on the channel, which by so much diminishes its obstructive 

 power: such reaction being shown in the motion acquired 

 by the detached portions which are carried away. The 

 cutting out of river-courses is a perpetual illustration of this 

 truth. Still more involved is the case of motion 

 passing through matter by impulse from part to part; as a 

 nervous discharge through animal tissue. Some chemical 

 change may be wrought along the route traversed, which 

 may render it less fit than before for conveying a current. 



