When specifying the great points of coincidence existing between the ordinary electric discharge and lightning, Franklin, as already partly stated, had remarked that flashes of lightning are frequently waving and crooked, of a zigzag or forked appearance, sometimes diffused and sometimes coloured (“On the Nature of Thunderstorms,” W. Snow Harris, London, 1843, p. 24; Priestley, “History and Present State of Electricity,” London, 1769, p. 166; “Encycl. Metropol.,” article “Electricity”; Biot, “Traité de Physique,” Vol. II). In treating of the subject of lightning flashes, Dr. L. D. Gale (trans. of M. F. J. F. Duprez’s paper on “Atmospheric Electricity,” taken from the memoirs of the Royal Academy of Brussels) alludes to the attempts made by C. G. Helvig to determine the velocity of the linear flashes (Gilbert’s Annalen, Vol. LI. pp. 136 and 139, ss. 2, 10) which he estimated to be 40,000 to 50,000 feet in a second, and states that M. Weigsenborn, of Weimar (Comptes Rendus, Vol. IX. p. 218), calculated the velocity of a flash observed in 1839 to be more than two leagues, while M. François Arago (“Annuaire,” etc., pour l’année 1838, pp. 249, 255, 257, 459, estimated the lengths of certain flashes to be 3·3, 3·6, 3·8 leagues. The views of Messrs. Logan (Phil. Trans., 1735, Vol. XXXIX. p. 240), L. J. Gay-Lussac (Ann. de Chim. et de Phys., 1805, Vol. XXIX. p. 105), H. W. Brandes (“Beiträge zur Witterungskunde,” etc., 1820, p. 353), C. H. Pfaff and L. E. Kaemtz (J. S. T. Gehler, “Dict. de Phys.,” Vol. I. p. 1001, and “Lehrbuch d. Meteor,” Vol. II. p. 430), Gabriel Lamé (“Cours. de Phys. de l’Ecole Polytech.,” Tome II. 2e partie, p. 82), Becquerel (Comptes Rendus, 1839, Tome VIII. p. 216), Faraday (Philos. Magazine, 1841, Vol. XIX. p. 104), Pouillet (“Eléments de Phys. et de Météor,” Tome II. p. 808), Parrot (J. S. T. Gehler, “Dict. de Phys.,” Vol. I. p. 999), are also set forth in the above-named translation of M. Duprez’s valuable work.
Humboldt informs us that “the most important ancient notice of the relations between lightning and conducting metals is that of Ctesias, in his Indica, Cap. IV. p. 169. He possessed two iron swords, presents from the King Artaxerxes Mnemon, and from his mother Parysatis, which, when planted in the earth, averted clouds, hail and strokes of lightning. He had himself seen the operation, for the king had twice made the experiment before his eyes” (“Cosmos,” Vol. II. N. 186). Ctesias was a man of great learning. He was a contemporary of Xenophon, and lived for a number of years at the Court of Artaxerxes Mnemon as private physician to the king. Diodorus states that Ctesias was highly honoured at the Persian court. An abridged edition of the Indica was printed by Stephens in 1594 (“Hist. Roy. Soc.,” C. R. Weld, London, 1848, Vol. II. p. 93; “La Grande Encyclopédie,” Vol. XIII. p. 536; “Biographie Générale,” Vol. XII. p. 568).
In imitation of Franklin, Doctor Lining, of Charleston, in South Carolina, sent a kite into a thunder cloud, and by that means dissipated the lightning (Philosophical Transactions for 1754, Vol. XLVIII. p. 757).
The opinion entertained by Franklin regarding the nature of electricity differs from that previously submitted by Dufay (A.D. 1733), in the manner shown by Noad at p. 6 of his Manual, London, 1859 edition.
What Dufay considered to be two distinct species of electricities, vitreous and resinous, Franklin conceived to be two different states of the same electricity, which he called positive and negative. This, which constitutes the foundation of the present theory of electricity, is usually called the Franklinian theory, but it can be said to belong equally to Dr. Watson, for he had communicated it to the Royal Society before Franklin’s opinion on the subject was known in England (Phil. Trans. for 1748, Vol. XLV. pp. 49, 491; Thomson, “Hist. Roy. Soc.,” p. 436). Noad, in paragraph 12, applies the latter theory to the case of a charged Leyden jar, alluding to Franklin’s discovery of the location of electricity in the jar, wherefrom is drawn the conclusion that it is upon the glass that the electricity is deposited, and that the conducting coatings serve “only, like the armature of the loadstone, to unite the forces of the several parts and bring them at once to any point desired” (see “Œuvres de Franklin,” trans. of Barbeu-Dubourg, Tome II. p. 16, 3e lettre).
Of his plus and minus theory, Franklin thus wrote to Mr. Collinson: “To electrise plus or minus no more needs to be known than this, that the parts of the tube or sphere that are rubbed do, in the instant of the friction, attract the electrical fire, and therefore take it from the thing rubbing; the same parts, immediately as the friction upon them ceases, are disposed to give the fire they have received to any body that has less.”
In an appendix to his official report as U.S. Commissioner at the Paris Universal Exposition of 1867, entitled “Franklin and Electrical Semaphores,” Professor Samuel F. B. Morse, LL.D., expressed himself as follows:
“It has frequently been asserted (on what authority I know not) that the first idea of an electric semaphore originated with Franklin. I have sought in vain in the publication of Franklin’s experiments and works for anything confirmatory of this assertion. On mentioning the subject to my friend Professor Blake, he kindly proposed examining the writings of Franklin in order to elicit the truth. From him I have received the following:
“‘I consulted several works for the purpose of ascertaining, if possible, the foundation for the statement that Franklin suggested the idea of semaphores by static electricity. I have not yet found any such suggestion, but I have noted that, following the experiments by Dr. Watson and others, in England, to determine the velocity of the electric discharge, and the time supposed to be required for the electrical discharges across the Thames, by which spirits were kindled, etc. (in 1747), Dr. Franklin (in 1748) made some similar experiments upon the banks of the Schuylkill, and amused his friends by sending a spark “from side to side through the river without any other conductor than the water” (vide Priestley’s “History of Electricity”). This was in 1748, at the end of the year. In 1756 “J. A., Esq.,” of New York (James Alexander), presented to the Royal Society a proposition “to measure the time taken by an electric spark in moving through any given space” by sending the discharge or spark down the Susquehanna or Potomac, and round by way of the Mississippi and Ohio rivers, so that the “electric fire” would have a circuit of some thousands of miles to go. All this was upon the supposition or assumption that the electric fire would choose a continuous water conductor rather than to return or pass through the earth. Franklin presented a paper in reply, in which he says “the proposed experiment (though well imagined and very ingenious) of sending the spark round through a vast length of space, etc. etc., would not afford the satisfaction desired, though we could be sure that the motion of the electric fluid would be in that tract, and not underground in the wet earth by the shortest way”’ (‘Franklin’s Experiments on Electricity, and Letters and Papers on Philosophical Subjects,’ 4to, London, MDCCLXIX, pp. 282, 283).
“Can it be possible that Franklin’s experiment of firing spirits and showing the spark and the effects of the electric discharge across the river originated, or forms the foundation for, the statement that he suggested the semaphoric use of electricity?”