1 90 THE WORLD MACHINE 



this motive force resides can be only the sun which, as the 

 regulating power of the universe, determines the distance and 

 the speed of each planet. Just as the moon moves round the 

 earth in virtue of the earth's attraction, so are all the planets 

 held in chains and kept in their orbits by the preponderant 

 force of the sun. You read and you gasp a little to find how 

 close he presses the quarry. 



Still, these general ideas are not sufficient ; he is a mathe- 

 matician as well as one of the earliest of modern physicists. 

 He has studied deeply the phenomena of light. He has found 

 that mathematical law determines distance and speed. Is it 

 possible that a mathematical law can express the force of this 

 attraction ? He has observed that light grows feebler the 

 greater the distance from the source of illumination. Con- 

 ceivably the motor force from the sun is weakened in the same 

 way. You perceive wherein he erred. 



If there be such a force acting outwards from the sun, it is 

 evident that it must extend in all directions that is, in the three 

 dimensions of space. It is not a question here simply of the 

 contour of a circle, but of the surface of a sphere, and since 

 spherical surfaces are to each other as the squares of their radii, 

 it follows that light grows feebler in proportion to the square 

 of the distance. It is astonishing, but this obvious fact escaped 

 Kepler and his time yet another illustration of the truth that 

 the human mind does not march ; it creeps. Bound up to 

 the erroneous idea of a simple proportion, Kepler concludes that 

 the attractive force of the sun is the same. Only a step separated 

 him from the law of inverse squares. 



He seeks even to identify this attractive force. Gilbert of 

 Colchester's great work, The New Physiology of the Magnet 

 (how quaint the title to our ears !), had appeared in 1600. 

 Kepler had read it with profound attention. Gilbert had com- 

 pared the earth to a huge magnet ; he surmised even that this 

 might be the attractive force of the planets ; he had said that 

 the earth and moon act towards each other like a pair of magnets. 

 Kepler takes one step further ; he likens the sun to a vast and 

 still more powerful magnet, and this magnetic force for him 

 was the secret of the sun's attraction. He even meets the 

 objection that if such a force existed, it would be cut off during 

 an eclipse. He points out that just as magnetism may act 

 through other solid bodies, it may act through the planets 



