-1859] 



WRITINGS OF JOSEPH HENRY. 



325 



the two cones h a c and d a e, or f 

 a g and h ai, which ( for a reason 

 similar to that given in the prece- 

 ding proposition) are respective- 

 ly equal to each other ; and as we 

 may consider the whole interior 

 surface of the spherical shell made 

 up of the opposite bases of a series 

 of pairs of similar cones, it is clear 

 that the particle at a will be equal- 

 ly attracted or repelled on all sides, 

 or in other words will be apparently un-affected by the action 

 of the excitement which may exist at the surface. 



From the first of these propositions, it 

 is easy to deduce the effect of a pointed 

 rod in discharging the electricity from a 

 globe. For if A, Fig. 7, be the centre 

 of a charged sphere, from which the 

 slender pointed conductor ah c projects, 

 then will the action of all the electrici- 

 ty of the sphere on the point a be the 

 sameasifdirected from the centre; and if 

 we suppose for example that the sphere 

 is charged with positive electricity, then 

 will the atoms of electricity of the point 

 a be repelled by all the atoms of the fluid of the globe, as if 

 they were concentrated at A, and also the atoms of the fluid 

 at the point 6, below a, will be repelled by all the atoms of 

 the electricity of the globe as if they were concentrated at 

 the same point, and so on with the atoms at c, &c. ; therefore 

 the atoms at the point a will not only be directly repelled out- 

 ward by the atoms of the fluid in the sphere, but they will 

 also be pressed outward by the repulsion exerted on each of 

 the atoms below, so that the whole force exerted to drive off 

 the fluid from the point a will be in some relation to the 

 number of atoms in the perpendicular column below this 

 point ; and hence the strong tendency to rupture the air and 

 to escape, which must exist in a point projecting from a 



I'la. 7. 



