THE DEVELOPMENT OF MECHANICS 249 



Wren and Halley as well as Hooke have worked it out that such 

 a force exists, and that it acts inversely as the square of the 

 distance. But all of them realise that they lack the decisive 

 proof. They do not as yet see how this theory can be brought 

 into agreement with Kepler's laws, as it must ; they have 

 many a debate on the subject. A few years later Hooke shows 

 a curious experiment with a pendulum, which he likens to a 

 planet going round the sun ; but the thing is to prove that the 

 path of the planet will be an ellipse. 



One day, doubtless as a jest, Wren offers Hooke and Halley 

 a book worth forty shillings if they bring him the demonstration 

 within two months. Hooke declares that he has it, but he 

 withholds his proof. Since it is not forthcoming, young Halley 

 finally travels down to Cambridge to have a talk with one of 

 the members of the Society there, a Dr. Newton, Lucasian 

 professor of mathematics, a retiring and secretive man, who 

 had already sent many interesting communications to the 

 Society, his newjly invented telescope as well. 



There Halley finds that the whole subject had been worked 

 out nearly twenty years before. Such, at least, is the accepted 

 tradition. The story is so extraordinary that it is worth in- 

 quiring a little as to whether the tradition will altogether hold. 



