The results achieved were wholly of a practical kind. With that strong common sense which formed the most marked feature of his character, Benjamin Franklin, at a very early period of his experiments, came to the conclusion that of the actual _nature_ of electricity we know nothing, and, in all probability, never can know anything, with our finite senses. But, never losing sight of this starting point, he treated electricity as astronomers do the movement of the heavenly bodies. Of the incomprehensible forces that keep countless worlds in their courses through measureless space, astronomers know no more than the most ignorant of mankind; still they are able to arrive at very accurate calculations concerning the directions followed by stars and planets, and the amount of time consumed in their wanderings through the inconceivable universe. To such astronomical endeavours Franklin limited all his researches, and it was precisely because he so limited them that he achieved greater successes than any other investigator of the phenomena of electricity. Together with many smaller matters, Benjamin Franklin added three great discoveries to the knowledge of electricity. The first was that the electric fluid--so called for want of a better word to express the action of the mysterious force--will run its course more easily and quickly through sharply pointed metals than in any other way. This had never before been demonstrated, nor, probably, been ascertained. The second great discovery of Franklin was that of positive and negative electricity, or, as he called it for some time, _plus_ and _minus_, the latter names being really the most descriptive. Of the actual existence of these two divisions of the great and marvellous agency, now attracting and now repelling each other, much was known previous to Franklin, but he was the first to make them clearly understood, and to bring their effects within reach of calculation. To these two discoveries Benjamin Franklin added a third, the greatest of all. He established the identity between the electric force and lightning, and upon it based one of the noblest inventions of all ages, that of the lightning conductor. And perhaps there never was any invention acknowledged more deeply by mankind. The French Academy expressed it when, on Franklin’s entrance, all the members rose, and the President exclaimed ‘_Eripuit cœlo fulmen_.’