254 ELEMENTARY LESSONS ON [CHAP. iv. 



of continual slight electrification in the air, best observed 

 when the -weather is fine. The phenomena of the Aurora 

 constitute a third branch of the subject. 



3O2. The Thunderstorm an Electrical Pheno- 

 menon. The detonating sparks drawn from electrical 

 machines and from Leyden jars did not fail to suggest 

 to the early experimenters, Hawkesbee, Newton, Wall, 

 Nollet, and Gray, that the lightning flash and the thunder- 

 clap were due to electric discharges. In 1749, Ben- 

 jamin Franklin, observing lightning to possess almost 

 all the properties observable in electric sparks, 1 suggested 

 that the electric action of points (Art. 43), which was 

 discovered by him, might be tried on thunderclouds, 

 and so draw from them a charge of electricity. He 

 proposed, therefore, to fix a pointed iron rod to a high 

 tower. Before he could carry his proposal into effect, 

 Dalibard, at Marly-la-ville, near Paris, taking up Franklin's 

 hint, erected an iron rod 40 feet high, by which, in 1752, 

 he succeeded in drawing sparks from a passing cloud. 

 Franklin shortly after succeeded in another way. He 

 sent up a kite during the passing of a storm, and found 

 the wetted string to conduct electricity to the earth, and 

 to yield abundance of sparks. These he drew from a 

 key tied to the string, a silk ribbon being interposed 

 between his hand and the key for safety. Leyden jars 

 could be charged, and all other electrical effects pro- 

 duced, by the sparks furnished from the clouds. The 

 proof of the identity was complete. The kite experi- 

 ment was repeated by Romas, who drew from a metallic 



1 Franklin enumerates specifically an agreement between electricity and 

 lightning in the following respects : Giving light ; colour of the light ; 

 crooked direction ; swift motion ; being conducted by metals ; noise in 

 exploding ; conductivity in water and ice ; rending imperfect conductors ; 

 destroying animals ; melting metals ; firing inflammable substances ; sul- 

 phureous smell (due to ozone, as we now know) ; and he had previously found 

 that needles could be magnetised both by lightning and by the electric spark. 

 He also drew attention to the similarity between the pale-blue flame seen 

 during thundery weather playing at the tips of the masts of ships (called by 

 sailors St. Elmo's Fire), and the "glow" discharge at points. 



