Mathematics and General Pltysics. 358 



ferent forms, or of different sizes. lie illustrated his position 

 by reference to the loaded Leyden jar, which receives exactly 

 the same charge, whether it is in part filled with a metallic body, 

 or is simply coated with a film of metal on its inside. He also 

 pointed out the fact, that the quantity, or size, or shape of the 

 metallic rods outside the jar, has no influence on the charge 

 which is found concentrated on that part of its surface which is 

 nearest the glass. Mr H. applied the same principles to explain 

 the nature of the charge on air. Several members expressed 

 their opinions for and against both views. It was admitted that 

 Coulomb's theory of diffusion is true in a perfectly imaginary 

 case, but that, in practice, it never can be exactly true, on ac- 

 count of the pressure of surrounding bodies, all of which exer- 

 cise a greater or less influence in concentrating the electric fluid 

 on that portion of the surface whic h is opposite to each. 



The charge of the intervening plate, it was observed, was in- 

 dependent of the conductors, and the amount of electricity in a 

 body cannot be ascertained but by bringing another body to- 

 wards it. Hence the results are uncertain. Mr Whewell re- 

 plied, that the same results were obtained when the connecting 

 body was different ; and, therefore, the experiment was not lia- 

 ble to uncertainty. 



2. Mr S. Harris explained and exhibited to the Section a new- 

 species of instrument of extreme delicacy, for measuring electrical 

 forces. It is of the same kind as the torsion balance of Cou- 

 lomb, and similar to it in general appearance, but the peculiarity 

 is this: — instead of a needle suspended by a single wire or fibre, 

 which fibre undergoes the torsion against which the antagonist 

 force of the electrical repulsion is to act, the needle is here sus- 

 pended by two parallel fibres run together, one on each side of 

 its centre ; and the effect may be most shortly described, by 

 saying, that neither of the fibres undergo torsion, but the plane 

 (in which they both lie when at rest) undergoes torsion^ or is 

 twisted into a surface of double curvature. The degree to 

 which this is carried, is here counterbalanced by a weight; or 

 gravity thus constitutes the antagonist force, instead of the re- 

 sistance of the fibre to torsion, in Coulomb's construction. It is 

 difficult to give the contrivance a good descriptive name, since 



