120 ELECTRICITY. 



] 749 ; that the paper now referred to was first written in the former year, but 

 that it was enlarged and improved and sent to England in 1749, which must, 

 therefore, be taken as its date. In this letter he enters very fully into his rea- 

 sons for considering the cause of electricity and lightning to be the same phys- 

 ical agent, differing in nothing save the intensity of its action ; and he truly 

 observes, that the difference in degree, however enormous, is no argument 

 against the identity of the agents, but that, on the contrary, an almost infinite 

 difference might be naturally looked for. " When a gun -barrel in electrical 

 experiments has but little electrical fire in it, you must approach it very near 

 with your knuckle before you can draw a spark. Give it more fire, and it will 

 give a spark at greater distance. Two gun-barrels united, and as highly elec- 

 trified, will give a spark at a still greater distance. But if two gun-barrels 

 electrified will strike at two inches distance, and make a loud snap, to what a 

 great distance may ten thousand acres of electrified cloud strike and give its 

 fire, and how loud must be that crack !"* 



The analogies which he stated as affording presumptive evidence of the 

 identity of lightning and electricity may be briefly enumerated. The electrical 

 spark is zigzag, and not straight ; so is lightning. Pointed bodies attract elec- 

 tricity ; lightning strikes mountains, trees, spires, masts, and chimneys. When 

 different paths are offered to the escape of electricity, it chooses the best con- 

 ductor ; so does lightning. Electricity fires combustibles ; so does lightning. 

 Electricity fuses metals ; so does lightning. Lightning rends bad conductors 

 when it strikes them ; so does electricity when rendered sufficiently strong. 

 Lightning reverses the poles of a magnet ; he proved by direct experiment that 

 electricity had the same effect. A stroke of lightning when it does not kill, c 

 often produces blindness ; he rendered a pigeon blind by a shock of electricity 

 intended to kill it. Lightning destroys animal life ; he killed a hen and a tur- 

 key by electrical shocks. 



Having ascertained by experiment the property of points in attracting and 

 discharging electricity, Franklin, acknowledging his inability to give a satis- 

 factory theory of this effect, set himself to inquire how " this power of points 

 might possibly be of some use to mankind." To discover this, he suspended 

 a large conductor, by silk lines, from the ceiling, and charged it with electricity, 

 so as to enable it to give a spark at the distance of two inches, " strong enough 

 ^ to make one's knuckle ache." Under these circumstances, he found that if a 

 I person presented the point of a needle to the conductor at more than a foot 

 1 distance, no electricity could be retained upon it, all passing off by the needle 

 as fast as it was supplied. He also found, that if, after it was strongly electri- 

 fied, the needle was presented at the same distance, the conductor would in- 

 stantly lose its electricity. That the electricity, in this case, really passed off 

 by the point, he ascertained by observing that, in the dark, the light was visi- 

 ble on the point of the needle ; and also because, when the person presenting 

 the needle was himself insulated, or stuck the needle in a bundle of sealing 

 wax, the electricity no longer escaped. 



The next experiment is so remarkable in itself, and so characteristic of the 

 mind of Franklin, that we shall give it in his own words : 



" Take a pair of large brass scales, of two or more feet beam, the cords of 

 the scries being silk. Suspend the beam by a packthread from the ceiling, so 

 that the bottom of the scales may be about a foot from the floor ; the scales 

 will move round in a circle by the untwisting of the packthread. Let an iron 

 punch (a silvfrsinith's iron punch, an inch thick, is what I use) be put on the 

 end upon the floor, in such a place as that the scales may pass over it in ma- 

 king their circle ; then electrify one scale by applying the wire of a charged 



* Letters, p. 218. 



