MOTION. ij 



body, or pressed by a spring which has previously been moved, 

 but to motion caused by attractions such as magnetism or 

 gravitation. Suppose a piece of iron at rest in contact with 

 a magnet at rest ; if it be desired to move the iron by the 

 attraction of the magnet, the magnet or the iron must first be 

 moved ; so before a body falls it must first be raised. A 

 body at rest would therefore continue so for ever, and a body 

 once in motion would continue so for ever, in the same direc- 

 tion and with the same velocity, unless impeded by some other 

 body, or affected by some other force than that which origi- 

 nally impelled it. These propositions may seem somewhat 

 arbitrary, and it has been doubted whether they are necessary 

 truths ; they have for a long time been received as axioms, 

 and there can at all events be no harm in accepting them as 

 postulates. It is however very generally believed that if the 

 visible or palpable motion of one body be arrested by its 

 impact on another body, the motion ceases, and the force which 

 produced it is annihilated. 



Now, the view which I venture to submit is, that force 

 cannot be annihilated, but is merely subdivided or altered in 

 direction or character. First, as to direction. Wave your 

 hand : the motion, which has apparently ceased, is taken up 

 by the air, from the air by the walls of the room, &c., and so 

 by direct and reacting waves, continually comminuted, but 

 never destroyed, It is true that, at a certain point, we lose 

 all means of detecting the motion, from its minute subdivision, 

 which defies our most delicate means of appreciation, but we 

 can indefinitely extend our power of detecting it accordingly 

 as we confine its direction, or increase the delicacy of our 

 examination. Thus, if the hand be moved in unconfined air, 

 the motion of the air would not be sensible to a person at a 

 few feet distance ; but if a piston of the same extent of sur- 

 face as the hand be moved with the same rapidity in a tube, 

 the blast of air may be distinctly felt at several yards' distance. 

 There is no greater absolute amount of motion in the air in 

 the second than in the first case, but its direction is restrained, 

 so as to make the means of detection more facile. By carry- 

 ing on this restraint, as in the air-gun, we get a power of 

 detecting the motion, and of moving other bodies at far greater 



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