"3rd. That long before it is exhausted the ingenuity of man will probably discover means of securing power from some other source than coal."
"What, for example?"
"Well, perhaps we shall learn how to utilize terrestrial magnetism directly. You know this earth of ours is a gigantic magnet, and magnetism is the raw material of electricity, if I may so express it. At present we get all the electricity we use out of the earth, but we have to do it by burning coal to run dynamos. Perhaps we shall find ways to save that expense by drawing the electricity directly from the earth. We have already done something closely resembling that, with the result of a great saving."
"How was that?"
"Why when the telegraph was first invented it was necessary to double the wire lines, putting up two wires every time by way of completing the circuit. You know electrical energy will not manifest itself, or as we say, the electric current will not flow, unless there is a circuit established. Well at first they established the circuit by running two parallel wires, one to carry the current one way and the other to bring it back. That's a clumsy way to put it, but it will answer my purpose in explanation. After a while somebody found out that the earth is a better conductor of electricity than any wire could be, and so the circuit was established simply by running each end of a single wire into the ground, making the earth do the work formerly done by the other wire. That simple discovery saved exactly one half the expense of telegraph companies for wires."
By this time it was growing late and as the boys had a hard morrow's work before them the Doctor ceased talking and all went to their bunks.
CHAPTER XLII
In the Service of the King
Very early the next morning the boys, who had caught the Doctor's enthusiasm, began again their task of digging through the "out crop" coal, which began now to grow softer and more workable, while the coal itself grew steadily better in quality.