ELECTRICITY IK MOTION. 27 



substances is so rapid, as to be performed in all cases without a 

 sensible interval of time. 1 h^s indeed b'ii said, that when very 

 weakly excited, and obliged to pass to a v, ry srreat distance, a 

 perceptible portion of time is actually occupied in i's passage ; but 

 this fact is somewhat doubtful, and attempts have been made, in 

 vain, to estimate the interval employed in the transmission of a 

 shock through several miles of wire. WP are not to imagine that 

 the same particl- s of the fluid which enter at one part, pass through 

 the whole condu -ting substance, any more than that the same por. 

 tion of blood which is thrown out of the heart, in each pulsation, 

 arrives at the wrist at the instant that the pulse is felt there. The 

 Telocity of the transmission of a spark, or shock, far exceeds the 

 actual velocity of each particle, in the same manner as the velocity 

 of a wave exceeds that of the particles of water concerned in its 

 propagation ; and this velocity must depend both on the elasticity 

 of the fluid, and on the force with which it is confined to the con. 

 ducting substance. If this force were merely derived from the 

 pressure of the atmosphere, we might infer the density of the fluid 

 from the velocity of a spark or shock, compared with that of 

 sound ; or we might deduce its velocity from a determination of 

 its density. It hns been supposed, although perhaps somewhat 

 hastily, that the actual velocity is nearly equal to that of li^ht. 



Wh*n a conducting substance approaches another, which is 

 electrified, the distribution of the electric fluid within it is neces- 

 sarily alt' red by induction, before it receives a spark, so that its 

 remoter extremity is brought into a state similar to that of the first 

 body : hence it ha| pens that when the spark passes, it produces 

 less effect at the remoter end of the sub-tauce, while the part pre- 

 sented to the electrified oody is most affected, on account of its 

 sudden change to an opposite state. But if both ends approach 

 bodies in opposite states oi electricity, they will both Le Mrongly 

 affected when the shock t;ik-s place, while the middle of the cir- 

 cuit undergoes but lit'le change. 



The manner in which the electric fluid makes its way through a 

 more or less perfect conductor, is not completely understood : it 

 is doubtful whether the substance is forced away on each side, so 

 as to leave a vacuum for the passage of the tiuid, or whether the 

 newly formed surface helps to guiue it in its way; and in some 

 cases it has been supposed, that the gradual communication of elec- 

 tricity has rendered the substance more capable of conducting it, 



