196 POPULAR LECTURES AND ADDRESSES. 



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he learned from accurate observations, and from the 

 earth's dimensions, what I have mentioned as the 

 moon's distance, and perceived the law of variation 

 between the weight of a body at the earth's surface 

 and the force that keeps the moon in her orbit. 

 The moon in Newton's theory was always falling 

 towards the earth. Why does it not come down ? 

 Can it be always falling and never come down ? 

 That seems impossible. It is always falling, but it 

 has also a motion perpendicular to the direction in 

 which it is falling, and the result of that continual 

 falling is simply a change of direction of this 

 motion. 



It would occupy too much of our time to go into 

 this theory. It is simply the dynamical theory of 

 centrifugal force. There is a continual falling 

 away from the line of motion, as illustrated in a 

 stone thrown from the hand describing an ordinary 

 curve. You know that if a stone is thrown hori- 

 zontally it describes a parabola the stone falling 

 away from the line in which it was thrown. The 

 moon is continually falling away from the line in 

 which it moves at any instant, falling away towards 

 the point of the earth's centre, and falling away 

 towards that point in the varying direction from 

 itself. You can see it may be always falling, no\v 

 from the present direction, now from the altered 

 direction, now from the farther altered direction in 

 a further altered line ; and so it may be always 

 falling and never coming down. The parts of the 



