1 86 THE WORLD MACHINE 



first bit of writing was on the double motion of the earth. Soon 

 he got a professorship at the little University of Gratz. 



All the time his teeming head was busy with a problem that 

 had got into it and would not out. This was as to the cause 

 of the motion of the planets, and why there are just six. He 

 makes what he thinks is a great discovery a purely fanciful 

 one, however that the number is due to the fact that there 

 are just five regular solids, and this obviously corresponds to 

 the gaps between the six planets ! His idea is in thorough 

 keeping with the time ; it is a survival of the old Pythagorean 

 mysticism of numbers. He writes a book about it, which con- 

 tains, for the rest, some curious speculations about a force 

 emanating from the sun which may be responsible for planetary 

 motion. 



The little volume wins for him the friendship of Galileo, 

 then a rising young professor at the old University of Padua. 

 It likewise brings him into communication with Tycho Brahe, 

 a Danish nobleman turned astronomer, and greatest scientific 

 figure of the age. Kepler is a convinced Coppernican. Tycho 

 is not, and will not be so long as he lives ; instead, he devises 

 a system of his own. Full of his theories, Kepler goes to see 

 him, and they discuss a great difficulty. 



Coppernicus had swept away the clumsy system of epicycles 

 handed down from Ptolemaic days. He made the planets move 

 in circles ; but it was plain as a pikestaff that they did not. 

 Tycho was one of the greatest observing astronomers that ever 

 lived the Herschel of his time. His observations made it all 

 too clear that the planets would not always appear where the 

 simple theory of circles supposed them to be. 



If there be any one who would know how great discoveries 

 are made, let him listen to the dictum of Newton : "I intended 

 my mind." Upon this problem Kepler intended his mind for 

 eight long years. He tried every device under the sun; none 

 of them would work. One day he thought to see if the planetary 

 motions would fit into an ellipse ; he sought after these things 

 with that almost insane intensity that some men seek gold ; 

 his joy at finding that he had hit the truth at last was almost a 

 delirium. 



Even before this his meditations and endless, ceaseless cal- 

 culating had disclosed one great law. This was that a line 

 joining any planet and the sun the radius vector, as the 



