324 



WRITINGS OF JOSEPH HENRY. 



[1855- 



In this figure, a represents a par- 

 ticle of matter or of electricity at- 

 tracted or repelled by the hollow 

 sphere of which the centre is C. Let 

 the two lines a d and a e represent 

 the projection of a pyramid hav- 

 ing its apex in a, and its base in 

 d e, then it will be evident that 

 the attraction of the three sections 

 of the cone, one through the cen- 

 tre, another coinciding with the 

 upper part of the spherical shell, 

 and the third with the lower part 

 included within d e, will be equal. 

 For although the lower section is 

 at a greater distance from a than 

 the upper, yet its greater size just compensates for the greater 

 distance, the surface increasing, as in the case of light, as the 

 square of the distance, while the attraction and repulsion 

 diminish in the same ratio. For the same reason, each of the 

 two portions of the spherical shell are equal in action to a plate 

 of equal thickness through the centre, included within the 

 cone ; and hence, the two together will be equal to a plate 

 of double thickness at the centre. 



If in the same way we suppose the whole spherical shell 

 included in a series of pyramids or cones, having as a common 

 apex the point a, and consider this series of cones made up 

 of equi-angular pairs, the two members of which are on each 

 side of the line through the centre as li a i, and/ a g, then it 

 will be clear that the resultant action of each of these pairs of 

 cones will be in a line through the centre, and all the action 

 of the sphere made up of such cones the same as if it were 

 at this point. 



That a point at the centre of a hollow sphere would be 

 equally acted upon in all directions is evident; but that the 

 same should be the case when the point is at a, Fig. 6, for 

 example, is not quite so clear. It may however be rendered 

 evident by considering the actions of the opposite bases of 



