1894.1 [,Mr. C. Vernon Boys on the Newtonian Constant, dtc. 353 



WEEKLY EVENING MEETING, 



Friday, Juno 8, 1894. 



Sir Frederick Abel, Bart. K.C.B. D.C.L. LL.D. F,R.S. 

 V ice-President, in the Chair. 



C. Vernon Boys, Esq. F.E.S. A.ft.S.M. M.R1. 



The Newtonian Constant of Gravitation. 



It is probably within the knowledge of most of those present that Sir 

 Isaac Newton, by his great discovery of gravitation and its laws, was 

 able to show that a single principle, ideally simple, viz. that every 

 particle in the universe attracts any other particle towards itself with 

 a force which is proportional to the product of their masses divided 

 by the square of the distance between them, would completely and 

 absolutely account for the three laws of planetary motion which Kepler 

 had given to the world. 



Newton also showed that a spherical body, whether uniformly 

 dense or varying in density according to any law from the centre to 

 the surface, would attract bodies outside with the same force that it 

 would do. if it could all be concentrated at its centre, i.e. that all the 

 attractions varying in amount and direction produced by particles in 

 all parts of a sphere need not be considered separately, but may be 

 treated in this simple way.* 



Nevertheless, though Newton's great discovery is sufficient to bring 

 the whole of the movements of the planets and of their satellites, 

 whether their simple Keplerian motions or the disturbances produced 

 by their mutual gravitation, the motions of comets, of binary stars, of 

 the tides, or the falling apple, under the domain of a single and simple 

 principle, though it enables one to compare the masses of the sun, the 

 planets and their satellites, and of those binary stars whose jjarallax 

 has been determined, one thing can never be made known by astro- 

 nomical research alone, though we may know that twenty-eight suns 

 would be required to make one Sirius ; that the sun is equal to 1018 

 Jupiters, that Jupiter is more than double all the rest of the solar 

 system put together, or that the moon is 1/80 of the earth ; no obser- 

 vations of these bodies can ever tell us how many tons of matter go to 

 make up any one of them. 



Though we know from first principles of dynamics, by the mere 



* Only last night I learned that it was the difficulty of proving this, and 

 not the erroneous value of the moon's distance, that delayed the publication of 

 Newton's discovery for so long. 



Vol. XIV." (No. 88.) 2 b 



