The to-and-fro nature of the discharge was thought, at a time, to account satisfactorily for the burr raised on each side of the pasteboard; but Trowbridge, of Harvard, has shown that even a unidirectional discharge, such as can be obtained by inserting a wet string or any high resistance in the circuit, would produce a double burr, from which we infer, confirming Franklin, that this effect of the discharge is caused by the sudden expansion of air within the paper itself. By the year 1749, Franklin had reached the conclusion that the lightning of the skies is identical with that of our laboratories, basing his belief on the following analogies which he enumerates in the notes or "minutes" which he kept of his experiments: "The electric fluid agrees with lightning in these particulars: (1) Giving light; (2) color of the light; (3) crooked direction; (4) swift motion; (5) being conducted by metals; (6) crack or noise in exploding; (7) rending bodies it passes through; (8) destroying animals; (9) melting metals; (10) firing inflammable substances; and (11) sulphurous smell." But although he felt the full force of the analogical argument, Franklin knew that the matter could not be finally settled without an appeal to experiment; and accordingly he adds: "The electric fluid is attracted by points; we do not know whether this property is in lightning. But since they agree in all the particulars wherein we can already compare them, is it not probable that they agree likewise in this? Let the experiment be made." (1749.) In writing to Collinson in July, 1750, he tells his London friend how the experiment may be made: "On the top of some high tower or steeple, place a kind of sentry-box--big enough to contain a man--and an electrical stand. From the middle of the stand let an iron rod rise and pass, bending out of the door, and then upright 20 or 30 feet, pointed very sharp at the end. If the electrical stand be kept clean and dry, a man standing on it, when such clouds are passing low, might be electrified and afford sparks, the rod drawing fire to him from the cloud." Collinson brought some of Franklin's letters to the notice of fellow-members of the Royal Society with a view to their insertion in the _Philosophical Transactions_ of that learned body; but even his epoch-making letter to Dr. Mitchell, of London, on the identity of lightning and electricity, was dismissed with derisive laughter. The Royal Society made amends in due time for their contemptuous treatment of the American philosopher by electing him member of the Society and by awarding him the Copley medal in 1753. Disappointed as he was, Collinson collected Franklin's letters and published them under the title of _New Experiments and Observations on Electricity made at Philadelphia in America_. The pamphlet appeared in 1751, and was immediately translated into French by M. d'Alibard at the request of the great naturalist Count de Buffon.