316 ELECTEICAL INTENSITY. [SECT, xxvni. 



farther without the sphere of each other's influence. It is 

 quite independent of the extent of the edge, the area being 

 the same ; for Mr. Snow Harris found that the electrical 

 intensity of a charged sphere is the same with that of a 

 plane circular area of the same superficial extent, and that 

 of a charged cylinder the same as if it were cut open and 

 expanded into a plane surface. 



The same able electrician has shown, that the attractive 

 force between an electrified and a neutral uninsulated body 

 is the. same, whatever be the forms of their unopposed parts. 

 Thus two hemispheres attract each other with precisely the 

 same force as if they were spheres ; and as the force is as 

 the number of attracting points in operation directly, and as 

 the squares of the respective distances inversely, it follows 

 that the attraction between a mere ring and a circular area 

 is no greater than that between two similar rings, and the 

 force between a sphere and an opposed spherical segment 

 of the same curvature is no greater than that of two simi- 

 lar segments, each equal to the given segment. 



Electricity may be accumulated to a great extent in insu- 

 lated bodies ; and, so long as it is quiescent, it occasions no 

 sensible change in their properties, though it is spread over 

 their surfaces in indefinitely thin layers. When restrained 

 by the non-conducting power of the atmosphere, the tension 

 or pressure exerted by the electric fluid against the air 

 which opposes its escape is in the ratio compounded of the 

 repulsive force of its own particles at the surface of the stra- 

 tum of the fluid, and of the thickness of that stratum. But, 

 as one of these elements is always proportional to the other, 

 the total pressure on every point must be proportional to 

 the squares of the thickness. If this pressure be less than 

 the coercive force of the air, the electricity is retained ; but, 

 the instant it exceeds that force in any one point, the elec- 

 tricity escapes, which it will do when the air is attenuated, 

 or becomes saturated with moisture. It appears that the 

 resistance of the air to the passage of the electric fluid is 



