Having improved in various ways the lightning conductor set up experimentally over his own house, Franklin erected a second one, of larger dimensions, to protect the residence of one of his friends, Mr. West, a wealthy merchant of Philadelphia. The apparatus, constructed entirely under the supervision of Franklin, consisted of an iron rod half an inch in diameter throughout its length, and ending at the bottom in a thick iron stake, driven four or five feet into the ground. The top of the conductor, rising nine feet above the central stack of chimneys, was formed by a brass wire ten inches in length, tapering off in a sharp point. Franklin considered the brass wire, which was screwed and soldered inside the iron rod, a great improvement upon simple iron, having discovered brass, as well as copper, to be better conductors of electricity. The result justified his expectations. Not many months after the lightning conductor had been erected over the mansion of Mr. West, a thunderstorm more severe than had been experienced for many years broke over Philadelphia. Vivid flashes of lightning followed each other incessantly, one of them striking, visible to all beholders, the house of Mr. West, touching the point of the conductor on the roof, and appearing again on its base in a thin sheet of flame. Naturally, Franklin was delighted at this first notable result of his grand discovery, and lost no time in examining the traces of the lightning over his conductor. He found that the sharp metal point at the upper end had been melted, and the small brass wire reduced from ten to seven and a half inches, with its top very blunt. The thinnest part of the wire, he saw at once, had disappeared in smoke, while the portion below it, a little thicker, had simply been liquefied, sinking down while in a fluid state, and forming a rough irregular cap, lower on one side than on the other. This was a highly interesting test, showing that the wire on the summit of the conductor must not be made too thin, so as to be liable to be burnt. But still more interesting to Franklin was the investigation of the report, confirmed on all sides, that a sheet of flame had been seen at the base of the conductor, where it was connected with the earth. He at once suspected that the earth at the point, and down to the end of the metal rod, had been very dry, and such indeed was the case. Hence he arrived at the conclusion that all conductors should go deep enough into the earth to find sufficient moisture quickly to dissipate the electric fluid. All subsequent experience, down to the present day, has proved that the inference of the practical philosopher of Philadelphia was as sound in this respect as in the rest of his ever clear and lucid judgments.