Page:Popular Science Monthly Volume 25.djvu/700

684 have effect in reference to all public buildings and churches. The report also became the chief authority on the subject in most foreign lands. It likewise served the useful purpose of weakening the opposition, which still endeavored to maintain that disastrous explosions were caused by conductors, and furnished clear and precise rules for construction that were intelligible to ordinary workmen.

In the year 1854 iron was much more generally used in buildings than it had been at an earlier date, and some additional knowledge of the conditions and laws of electrical action had been acquired. The Academy, on this account, thought it well to request the Section of Physics to reconsider the lightning-rod instruction of 1823. This led to the first report, which was prepared by M. Pouillet, adopted by the Academy of Sciences on March 5, 1855, and immediately afterward issued by the Government as an additional instruction. In this document it was held that the large masses of iron employed in buildings certainly serve to attract the lightning. If two buildings of an equal size were similarly placed, the one being exclusively of stone and wood, and the other having large masses of metal in its construction, the lightning would certainly strike the latter and avoid the former, just as, when a ball of metal and a ball of wood are presented together toward a charged prime conductor of an electrical machine, it is always the former, and never the latter, which receives the spark. A dry soil, it was pointed out, does not attract the lightning. But, if, under such a soil, there occur at some depth large masses of metal, or accumulations of water, the lightning would explode through the dry earth, splitting it up as a coat of varnish is pierced by an electric spark. The line of lightning-discharge is always marked out for it beforehand, in conformity with the law of electric tension, beginning at the same instant at both the extremities of the track. The objects which are most liable to strokes of lightning are good conductors that project farthest over toward the clouds.

In the report of 1855 the occasion was used to draw attention to some instructive instances of the mechanical effects of lightning-discharges which had taken place upon the open sea. In 1827 the packet-boat New York, not at the time carrying a conductor, was struck during its passage across the ocean, and a leaden pipe, three inches in diameter and one inch thick, was fused where the discharge escaped into the sea. A chain of iron wire, one quarter of an inch in diameter and one hundred and thirty feet long, having been then hoisted up on one of the masts and trailed in the sea, was struck by a second discharge, and scattered into molten molecules and broken fragments, the bridge being set on fire, although at the time covered by a sheet of hail and a deluge of rain. The Jupiter, in the North Sea fleet, in 1854, carrying a chain of several strands of fortieth-of-an-inch brass wire, two hundred and sixty feet long, hung from the mainmast-head, and trailing seven feet into the sea, was struck, and had the chain