a tunnel eight thousand miles long, and have begun work at both ends. Besides, I have only been able to allow a margin for error of three feet at the point of meeting. A margin of three feet on eight thousand miles is, as you can readily calculate for yourself, only a margin of one two hundredth of an inch on each mile! In other words, if, in digging my tunnel, I have made an error of one two hundredth part of an inch on each mile, or, at least, if these errors do not compensate, my tubes will not meet, and half the work will have to be done over again!"
"Whew!" whistled Mr. Curtis. "And you knew this when you started to build the tube?"
"Perfectly," said Dr. Giles; "and I was accordingly obliged to devise instruments of almost impossible delicacy of operation to use in aiming my tubes. The next twenty-four hours will show me whether or not my aim has been true, or whether we shall have five years more of weary working and waiting. Do you wonder now that I seem agitated, and that I cannot stay quietly in place, but pace up and down like a caged animal?"
