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longer in movement. A delicately suspended pendulum 

 is almost free from friction against its supports, but it is 

 gradually stopped by the resistance of the air ; place 

 it in the vacuous receiver of an air-pump and we find 

 the motion immensely prolonged. A large planet like 

 Jupiter experiences almost infinitely less friction, in 

 comparison to its vast momentum, than we can produce 

 experimentally, and we find through centuries that there 

 is not the least evidence of the falsity of the law. Expe- 

 rience, then, informs us that we may approximate indefi- 

 nitely to a uniform motion by sufficiently decreasing the 

 disturbing forces. It is a pure act of inference which 

 enables us to travel on beyond experience, and assert that, 

 in the total absence of any extraneous force, motion would 

 be absolutely uniform. The state of rest, again, is but a 

 singular case in which motion is infinitely small or zero, 

 to which we may attain, on the principle of continuity, by 

 considering successively cases of slower and slower motion. 

 There are many interesting cases of physical pheno- 

 mena, in which, by gradually passing from the apparent 

 to the obscure, we can assure ourselves of the nature of 

 phenomena which would otherwise be a matter of great 

 doubt. Thus we can sufficiently prove, in the manner of 

 Galileo, that a musical sound consists of rapid uniform 

 pulses, by causing strokes to be made at intervals which 

 we gradually diminish until the separate strokes coalesce 

 into a uniform hum or note. With great advantage we 

 approach, as Tyndall says, the sonorous through the 

 grossly mechanical. In listening to a great organ we 

 cannot fail to perceive that the longest pipes, or their 

 partial tones, produce a tremor and fluttering of the 

 building. At the other extremity of the scale, there is 

 no fixed limit to the acuteness of sounds which we 

 can hear ; some individuals can hear sounds too shrill for 

 other ears, and as there is nothing in the nature of the 



