THE TIDES. (APP. A.} 193 



to weigh out pounds of tea, to weigh them with 

 brass weights is to compare their gravitations 

 towards the earth to compare the heavinesses of 

 the different bodies. But the first subject that 

 I asked you to think of had nothing to do with 

 heaviness. The first subject was the mass of the 

 different bodies as tested by their resistance to 

 force tending to set them in motion. I may just 

 say that the property of resistance against being 

 set into motion, and again against resistance to 

 being stopped when in motion, is the property of 

 matter called inertia. 



The first great point in Newton's discovery 

 shows, then, that if the property of inertia is 

 possessed to an equal degree by two different 

 substances, they have equal heaviness. One of 

 his proofs was founded on the celebrated guinea 

 and feather experiment, showing that the guinea 

 and feather fall at the same rate when the resist- 

 ance of the air is removed. Another was founded 

 upon making pendulums of different substances 

 lead, iron, and wood to vibrate, and observing 

 their times of vibration. Newton thus dis- 

 covered that bodies which have equal heaviness 

 have equal inertia. 



The other point of the law of gravitation is, that 

 the force between any two bodies diminishes as 

 the distance increases, according to the law of the 

 inverse square of the distance. That law expresses 

 that, with double distance, the force is reduced one 



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