316 KELVIN 



pared with the other body, then the mass of the one is said 

 to be equal to the mass of the other. 



I have spoken of mutual forces between any two masses. 

 Let us consider the weight or heaviness of a body on the 

 earth's surface. Newton explained that the attraction of the 

 whole earth upon a body for example, this 56 pounds mass 

 of iron causes its heaviness or weight. Well, now, take 

 56 pounds of iron here, and take a mass of lead, which, 

 when put in the balance, is found to be of equal weight You 

 see we have quite a new idea here. You weigh this mass 

 of iron against a mass of lead, or to weigh out a commodity 

 for sale; as, for instance, to weigh out pounds of tea, to 

 weigh them with brass weights is to compare their gravita- 

 tions 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 resist- 

 ance 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 resistance of the air is re- 

 moved. Another was founded upon making pendulums of 

 different substances lead, iron, and wood to vibrate, and 

 observing their times of vibration. Newton thus discovered 

 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 quarter, at treble distance the force is 

 reduced to one-ninth part. Suppose we compare forces at 

 the distance of on million miles, then again at the distance 

 of two and a half million miles, we have to square the one 

 number, then square the other, and find the proportion of the 



