318 KELVIN 



the distance from the moon, we are led to the comparison 

 now indicated, but I am inverting the direction of Newton's 

 comparison. He found by observation that the moon falls 

 as far in an hour as a stone falls in a second, and hence 

 inferred that the force on the moon is a 6oth of the 6oth 

 of the force per equal mass on the earth's surface. Then 

 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 al- 

 ways 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 fall- 

 ing, 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 horizontally it describes a parabola the stone fall- 

 ing 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, 

 now from the present direction, now from the altered direc- 

 tion, now from the farther altered direction in a further 

 altered line ; and so it may be always falling and never com- 

 ing down. The parts of the moon nearest to the earth tend 

 to fall most rapidly, the parts furthest from the earth, least 

 rapidly; in its own circle, each is falling away and the 

 result is as if we had the moon falling directly. 



But while the moon is always falling towards the earth, 

 the earth is always falling towards the moon; and each 

 preserves a constant distance, or very nearly a constant 

 distance from the common centre of gravity of the two. 

 The parts of the earth nearest to the moon are drawn 



