"harnessed up" and pressed into service. This was the cheapest motive power that could be obtained, and it was withal efficient, easy to handle, and ample to perform many tasks like the one set it. As before mentioned, this power was converted into electricity and then carried down into the tube, along wires, to the points where it was required.
As the work advanced the difficulties increased. The greatest obstacle seemed to come from the internal heat of the earth; for in spite of what the doctor had said, the temperature rose with every mile's progress.
Mr. Curtis, who was always taking flying trips to the tube to see how matters were progressing and to give the doctor much unsought advice, was delighted when he noticed this steady increase of temperature.
"What did I tell you?" he cried, his eyes beaming with pleasure at the fulfilment of his prophecy. "I guess you'll soon have to set your refrigerating liquids circulating through the tube if you want to keep it from melting."
"I have given up that idea of pumping cold liquids through the tube," replied the doctor.
"The experiments I made in that line have con-
