ELECTRICITY. 503 



in the other, to effect a contact with the knob of the phial. 

 Whenever the discharges took place, the shocks were 

 felt by both persons : thus proving that the electric fluid 

 must have been in motion along the whole line of the 

 circuit, including both the wire above and the river 

 below. 



* In another experiment, made on Shooter's hill, at a 

 time when the ground was remarkably dry, the electricity 

 was made to perform a circuit of four miles ; being con- 

 ducted for two miles along wires supported upon baked 

 sticks, and for the remaining distance also of two miles, 

 through the dry ground. As far as could be ascertained, 

 by the most careful observation, the time in which the 

 discharge was transmitted along that immense circuit 

 was perfectly instantaneous : nor has any other trial that 

 has yet been made afforded the least approach to a mea- 

 surement of the velocity with which electiicity moves. 



'On this subject, however, an important distinction 

 should be made between the actual movement of each 

 individual particle of electric fluid, and the transmission 

 of an impulse along a series of such particles, for the 

 one may bear hardly any proportion to the other, just as 

 we find that sound proceeds with a velocity incompara- 

 bly greater than that of the particles of air which are 

 concerned in its propagation. In like manner the por- 

 tion of blood, which raises the artery at the wrist, where 

 the pulse is felt, is not the identical portion of blood, 

 which is thrown out from the heart by the contraction of 

 that organ producing that pulsation ; the impulse in all 

 these cases being propagated, like a wave, from one par- 

 ticle to another. There is, therefore, no reason to sup- 

 pose that the same particles of electric fluid, which en- 

 ter at one part, have traversed from one end to the other 

 the whole line of conducting substances.' 



2. It always chooses the best conductors which are in 

 its path. 



The distinction between conductors and non-conduc- 

 tors of the electric fluid has been already explained. 

 Some substances allow the passage of the electric fluid 

 with great ease; others with greater or less difficulty 



