The Professor significantly laid his hand upon the zinc tube which enveloped the flagstaff. "O ho!" cried the Doctor, "why did not I think of that?"
We should have explained before that the spiral stairs ran up between two zinc tubes, the one six feet in diameter, and the other two feet in diameter. The latter surrounded the mast, and after the globe should rise from the flagstaff this tube would indeed be a shaft two hundred feet in depth, or two hundred and ten feet, for it extended to the top of the roof of the observatory.
Accordingly, the burners were lighted, the globe arose until the ball of the mast was just below the level of the floor of the engine-room. Upon looking through the tube after all light had been excluded from the engine-room, a bright star could be seen shining down upon them with resplendent brilliancy.
"Now, Will," cried Professor Gray, "I wish you could go up and lower a plumb-line from the exact center of the top of the shaft. I want to see if our tube stands perpendicularly. If it does, and the plumb-line points straight through the center of it to yonder star, then we are at the exact spot we seek."
The line was lowered, and after a little adjustment of the cables, the lower end of the plumb-line passed through the exact center of the tube. The Professor ran his eye up the line and smiled with satisfaction.
"Look at it, Doctor," he said.
"Well, that is wonderful!" cried Dr. Jones. "Look at it Fred, Denison. The line runs precisely in alignment with the star."
"And now," said Professor Gray, after all had verified this last statement, "let's not lose a moment's time. Get your camera out. We want a twenty-four hours' exposure through our shaft, and photograph that star. If we be exactly at the Pole, it will describe a perfect circle upon the sensitive plate. If we are not so located, the line upon the plate will form an ellipse."
The camera was set as suggested by the Professor, and then the party retired for the night. We say "night," but the reader will constantly bear in mind that this term is not used with reference to daylight or darkness, simply to the clock, or time of day.
There was an absolute, dead calm during the following twenty-four hours after Will had set the camera. Nature was so extraordinarily kind to Dr. Jones during the time that we almost tremble for our reputation for veracity as we record the last-mentioned fact. Any swaying of the globe by the wind would have effectually prevented anything like a good negative being made. But the globe remained in the exact position, the atmosphere in the hot air chamber being kept up sufficiently so that a steady strain was maintained upon the four cables. At the end of the time mentioned the Professor examined the negative with a magnifying glass, and pronounced the test perfectly satisfactory.