In the history of human inventions and discoveries, the idea of the lightning conductor is almost the sole one which sprang, all but perfect, from one brain, like Minerva, in Greek mythology, from Jupiter’s head. Benjamin Franklin discovered the lightning conductor, and, except some important improvements in its manufacture, due to the progress of the metallurgical arts, the conductor remains the same, in essence, as designed by the world-famous citizen of Philadelphia. The reason of this is plain enough. Though one of the most brilliant discoveries in the annals of mankind, the lightning conductor, by itself, is one of the simplest of things. Franklin found by experiments, that the mysterious so-called ‘electric fluid’ had a tendency to make its way in preference through metals, and so he recommended the laying-down of a metallic line from the clouds to the earth to prevent damage to surrounding objects, such as buildings and the human beings within them. More than this he did not know; and more than this we, to this day, do not know. Of the inner nature, or constitution, of that grand cosmic discharge of electricity to which the name of lightning is given, no scientific explanation can be given. We are utterly ignorant of it, and in all probability ever will be. But while the general principle laid down by Franklin, that metals will conduct the electric force harmlessly from the clouds to the earth, remains the same, very much has been learnt, in the progress of scientific investigation, as regards the varying conducting capacity of different metals. The first conductors were invariably rods of iron, this metal being preferred by Franklin and his immediate followers as cheap, ready at hand, and answering all purposes in practice. But it was gradually found by experiments that there are other metals through which the electric force will make its way more rapidly than through iron. One of the earliest investigators of this subject was Sir Humphrey Davy, the celebrated inventor of the miner’s safety lamp. It was while studying the decomposition of the fixed alkalies by galvanism, and tracing the metallic nature of their bases, to which he gave the names of sodium and potassium, that the great chemist and natural philosopher was brought to enter upon an examination of what may be called the permeability of the different metals by the electric force. The result of his investigations, as stated by him, was that silver stood highest as a conductor of electricity; next to it coming copper; then gold; next, lead; then platinum; then the new metal called palladium--discovered by Wollaston, 1803, in platinum--and lastly, iron. These were the principal metals experimented upon by Sir Humphrey Davy, and the net result of his inquiries was expressed summarily in the fact of copper being more than six times, and silver more than seven times, as good a conductor as iron. Taking copper at 100, Sir Humphrey Davy drew up the following table of the electrical