The electric spark 8 1 



pressure of the electric fluid against the base of the cylinder is scarcely 

 greater than the force with which the two bodies attract each other, 

 provided that no part of the cylinder is undercharged; which is very 

 unlikely to be the case, if the electric repulsion is inversely as the square 

 of the distance, as I have great reason to believe it is ; and, consequently, 

 if the spark was produced by the air being pushed aside by the force with 

 which the fluid endeavours to issue from the cylinder, no sparks should 

 be produced, unless the electricity was so strong, that the force with 

 which the bodies attracted each other was as great as the pressure of the 

 atmosphere against the base of the cylinder: whereas it is well known, 

 that a spark may be produced, when the force, with which the bodies 

 attract, is very trifling in respect of that*. 



139] One may frequently observe, in discharging a Leyden vial, that 

 if the two knobs are approached together very slowly, a hissing noise will 

 be perceived before the spark; which shews, that the fluid begins to flow 

 from one knob to the other, before it passes in the form of a spark; and 

 therefore serves to confirm the truth of the opinion, that the spark is 

 brought about in the gradual manner here described. 



[* Note 10, p. 375.] 



C P. I. 



