96 GREAT MEN OF SCIENCE 



by Wilhelm Weber a hundred and forty years later. Hydro- 

 dynamics, developed further upon Newton's foundation, has 

 also succeeded in calculating the velocity of propagation for 

 any amplitude, but nothing of fundamental novelty concern- 

 ing these gravitational waves has been added. 



The wide scope of his Principia is already stated by Newton 

 himself in his mention of those parts of science which were 

 then only present in their beginnings, or where merely 

 indicated as possible, and of which he says that we have not 

 sufficient experience {Copia experimentorum), to be able to 

 say anything of a definite nature.^ He mentions the forces 

 with which neighbouring particles of a body attract one 

 another over the shortest distance, so as to hold the body 

 together; from which it appears that he did not regard these 

 forces-the molecular and the chemical forces in our present- 

 day terminology - as did Galileo and even Huygens, as 

 due to external pressure, but as acting like gravitation, but, 

 being different from it. He also mentions electrical attrac- 

 tion and repulsion acting over greater distances, light and 

 its phenomena and heating effects, and the phenomenon 

 of life. The latter phenomenon is one which we, two 

 hundred years later, still regard in all its aspects, with an 

 amazement, and practically complete ignorance, as great 

 as Newton's. 



This is true to almost the same extent concerning the 

 cause of gravitation {causum gravitationis nondum assignavi).^ 

 We only know as Newton did, that no matter is without it, 

 that it proceeds from every single particle of the whole 

 volume of every body, acts through everything, and main- 

 tains the law of inverse squares to the greatest distances; and 

 we are obliged - if we leave aside assumptions which are not 

 yet sufficiently tested - to continue to say with Newton: 

 'Enough that the existence of gravitation has become clear, 



* Conclusion of the Scolium generale at the end of the Principia. 

 ^ Scolium generale. 



