256 ^ PHILOSOPHICAL TRANSACTIONS. [aNNO 1771. 



some fluid to flow from the farthest part of a, where it is attracted with less 

 force, to the nearest part, where it is attracted with more force ; so that b will 

 attract the fluid in a with more force than it repels the matter. 



Case 5 and 6. If A is now supposed to be not insulated and not electrified, b 

 being still negatively electrified; it is plain that they will attract with more force 

 than in the last case : and if a is positively electrified, tliey will attract with still 

 more force. 



In these last 3 cases also, the efFect which a has in altering the quantity and 

 disposition of the fluid in b, tends to increase the force with which the two bodies 

 attract. 



Case 7. It is plain that a non-conducting body saturated with fluid, is not at 

 all attracted or repelled by an over or undercharged body, until, by the action ot 

 the electrified body on it, it has either acquired some additional fluid from the 

 air, or had some driven out of it, or till some fluid is driven from one part of the 

 body to the other. 



Case 8. Let us now suppose that the two bodies a and b are both positively 

 electrified in the same degree. It is plain, that were it not for the action of one 

 body on the other, they would both be overcharged, and would repel each other. 

 But it may perhaps be said, that one of them as a may, by the action of the 

 other on it, be either rendered undercharged on the whole, or at least may be 

 rendered undercharged in that part nearest to b ; and that the attraction of this 

 undercharged part on a particle of the fluid in b, may be greater than the re- 

 pulsion of the more distant overcharged part : so that on the whole the body a 

 may attract a particle of fluid in b. If so, it must be afiirmed that the body b 

 repels the fluid in a ; for otherwise, that part of a which is nearest to b could 

 not be rendered undercharged. Therefore, to obviate this objection, let the 

 bodies be joined by the straight canal dc of incompressible fluid (fig. 19). The 

 body B will repel the fluid in all parts of this canal ; for as A is supposed to attract 

 the fluid in b, b will not only be more overcharged than it would otherwise be, 

 but it will also be more overcharged in that part nearest to a, than in the oppo- 

 site part. Moreover, as the near undercharged part of A is supposed to attract 

 a particle of fluid in b, with more force than the more distant overcharged part 

 repels it ; it must, a fortiori, attract a particle in the canal with more force than 

 the other repels it ; therefore the body a must attract the fluid in the canal; and 

 consequently some fluid must flow from b to a, which is impossible; for as a 

 and B are both electrified in the same degree, they contain the same quantity of 

 fluid as if they both communicated with a third body at an infinite distance, by 

 canals of incompressible fluid ; and therefore by the corol. to prop. 24, if a com- 

 munication is made between them by a canal of incompressible fluid, the fluid 

 would have no disposition to flow from one to the other. 



