MOTOR FORCE OF THE WORLD MACHINE 261 



in the broadest sense he was the founder of molecular mechanics 

 as well. He gave, moreover, to general mechanics the last of 

 the great conceptions needed for the perfection of the science ; 

 he saw that action and reaction are equal. This, applied to 

 planetary motion, gave a complete explanation of the curious 

 perturbations of their orbital motion, long observed and mapped 

 out, but until then inexplicable. It was not merely that the 

 earth attracts the moon ; the moon in proportion to its mass 

 attracts the earth and gives to its daily revolution the form 

 of an eccentric. Could we watch it spin, we should see its axis 

 wobble about a centre just as does the eccentric of a steam- 

 engine. This same pull of the moon, exerted upon the unequal 

 shape of the earth, gives to the direction of its axis the peculiar 

 disturbance known as nutation, which will be discovered a few 

 years later by Bradley. The combined pull of the sun and the 

 moon, acting on this same uneven mass, produces the second 

 displacement of the earth's axis, to which the precession of the 

 equinoxes, discovered by Hipparchus, is due. 



Newton saw that the moon is attracted not only by the 

 earth but by the sun as well ; hence its orbit will not be a true 

 ellipse. By this means he explains and works out many of 

 the disturbances of the motion of the moon, the variations in 

 its inclination and the like. From the developments he had 

 given to Kepler's laws he perceives that, knowing the distance 

 of any body and the time of its revolution, he may calculate 

 its mass in terms of the central body ; he calculates the mass 

 of the moon, and, further than this, lays the foundations for 

 a second means of verifying the calculated distance of the sun 

 itself ; further yet, the mass of the sun. With this method, 

 from careful observations of the variations of the planets from 

 their natural paths as they react upon each other, it will one 

 day be possible for Leverrier to determine at what distance 

 the sun must be in order to produce the precise effects which 

 it does. 



Newton laid the foundations for a knowledge of the true 

 figure of the earth, and calculated that figure in its main details. 

 He foresaw from the observed effects that the earth could not 

 be a perfect sphere, that it was flattened at the poles, that its 

 equatorial diameter was greater than the polar diameter, and 

 that therefore the force of gravity at the poles was less than 

 at the equator. He thus pointed out the path whereby in after 



