It will be seen that, according to the investigations of Professor Matthiessen, admitted on all hands to be correct, the copper lowest in the list, the ‘Rio Tinto,’ is barely equal to iron in electrical conductivity, and, not having the hardness of the latter metal, would be in every way inferior to it as a lightning protector. The employment of the purest copper therefore became an essential point in the manufacture of lightning conductors. Fortunately, the difficulty was solved, at an earlier period than might have been expected, by the demand for submarine cables. These had to be made of wires of the highest possible electrical conductivity, and the matter being one of high financial and commercial importance, manufacturers soon began to use the utmost care in selecting ores containing the smallest amount of metallic impurities. We believe the lightning conductors now manufactured at the extensive works of Mr. R. S. Newall, F.R.S., established at Gateshead on Tyne about forty years ago, have generally a conductivity of 93 per cent. of pure copper. It was laid down by one of the most eminent scientific men of the day, not long ago, that the three principal qualities of a good lightning conductor ought to be a maximum of conductivity, of durability, and of flexibility that could be obtained, and there is nothing coming up to this standard so well as ropes of pure copper. CHAPTER X. HÔTEL DE VILLE, BRUSSELS, AND WESTMINSTER PALACE. The systems of lightning-conductors used for the protection of the Hôtel de Ville and Westminster Palace seem worthy of separate description, as showing the methods employed by Professor Melsens and the late Sir William Snow Harris, both eminent authorities in their respective countries. The two buildings are so entirely distinct in their character, that it will be seen at once that very different methods had to be employed in rendering them safe from the effects of thunderstorms.