9G MR. J. H. POYNTING ON THE LAW OF FORCE IN A SPHERICAL 



TABLE. 

 Art. 



20. 8-fold, 3 pair obv., all evenly di.st 4 



two evenly dist 8 



12 12 



4 p.iir obv 4 



78 

 Grand Total 



78 

 396 



XIII. On the Law of Force when a Thin, Homogeneous, 

 Spherical Shell exerts no Attraction on a Particle within 

 it. By J. H. PoYNTiNG, B.A., B.Sc. 



Read March 6th, 1S77. 



If a homogeneous, thin, spherical shell of uniform thick- 

 ness exert no attraction on a particle within it, then the 

 law of the force is the law of nature. 



Professor Maxwell uses this proposition {' Electricity/ 

 vol. i. § 74) to deduce the law of the force between electri- 

 fied bodies, and shows that it proves, far more conclusively 

 than any direct measurements of electrical forces, that the 

 law is that of the inverse square. It would therefore be 

 an advantage to have a simpler proof of such an important 

 proposition than that given by Laplace (Mec. Celeste, liv. 

 ii. ch. 2) and followed by Maxwell. The following seems 

 more simple, as it requires neither integration nor the so- 

 lution of a functional equation : — 



Let P be any point inside the spherical shell, C the 



