The Evolution of the Sciences 



This belief in inertia is the result of superficial ob- 

 servation. It is true that our piece of chalk will 

 remain on the table, when once placed there, 

 until some other moving body, say a hand, comes 

 into contact with it, and that once thrown into 

 space it will continue its motion after the pro- 

 pelling agent has ceased to act. It is again 

 inertia which compels the mass of the pendulum 

 to reascend the second part of its trajectory. 

 Likewise, if we open suddenly the tap of a water- 

 pipe the liquid does not immediately reach its 

 normal speed, and if we shut the tap just as 

 suddenly, the liquid, carried ahead by its own 

 movement, will strike against and perhaps break 

 the pipe; this is the water hammer familiar to 

 students of hydraulics. All these actions are 

 co-ordinated and explained ingeniously by the 

 principles of mechanics founded on the existence 

 of forces and on the inertia of matter. We may 

 be sure that these ideas, on which the human 

 mind has rested for centuries, are far from being 

 valueless, and do not deserve our disdain, but 

 we must not allow the force of habit to trans- 

 form simple hypotheses into indisputable prin- 

 ciples. We must rather recognise that the world 



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