Page:Popular Science Monthly Volume 25.djvu/702

686 alike to ascertain what it is that lightning spares, as well as what it strikes. It is of the utmost importance, for the advance of man's knowledge in this branch of physical investigation, that all instances of injury from lightning should be immediately examined and tested, and that all facts ascertained should be accurately described and placed upon record.

In the year 1866 the Minister of War in France became doubtful in regard to the measures which were then taken to secure powder-magazines against accident from lightning, and in consequence once again brought the matter formally under the consideration of the Academy of Sciences. It was this action of the minister which led to the third report, also drawn up by M. Pouillet, adopted by the Academy in the beginning of 1867, and shortly afterward issued under the authority of the French Government. In this report the best method of making joints in a conductor by overlapping, riveting, and soldering the contiguous ends, was pointed out, and it was urged that the underground continuation of the rod should be carried on to an adequately moist place, even if miles had to be traversed for the purpose. The increase in the number of air-terminals and the connecting them together were deemed of more consequence than the increasing the height of a smaller number. Secondary terminals were advised for every additional length of thirty-three yards of roof. The expansion of rods by heat was provided for by inserting free semicircular bands of red copper at suitable intervals, four inches of addition to the length being allowed for in every hundred yards of rod.

The example set by France in the preparation of these reports was followed for the first time in England by the appointment of a Naval Commission in 1839 to inquire into the protection of the vessels of the Royal Navy. This commission was formed in consequence of the public attention which had been drawn to the matter by Snow Harris, who stated that, within the forty years that ended in 1832, two hundred and fifty vessels had been more or less seriously injured by lightning. The commission somewhat haltingly reported that there was no harm in lightning-conductors, and that it thought the system of protection might be tried. Snow Harris thereupon introduced the plan of nailing a double set of overlapping strips of copper along the masts. After the adoption of this method the conductors were struck by lightning in several instances, but in no case did the vessels suffer any damage. This excellent system was only superseded in the end by the natural result of the introduction of iron vessels, which made the ships themselves efficient conductors in virtue of the principle of their construction. The original idea of Snow Harris was, indeed, to bring the general structure requiring defense as nearly as possible into the same non-resisting state that it would have if entirely composed of metal. He was knighted for his services in 1847, and in 1855 was employed to design the protection of the then new Houses of Parliament at