"Yes," said Don, picking up the marine glasses and gazing intently to the westward behind them, "and it looks as though they were going to come mighty near doing it, too. Great Scott, Jack! take those glasses and look at that burst of speed."
He handed the glasses to Big Jack, while the others also turned their gaze to where the naked eye could just discern a slowly enlarging speck over the western horizon.
For a full minute Jack remained with the glasses to his eyes. Then he turned to look at his own air-speed indicator.
"We're doing a hundred and fifteen," he announced. "We'll put her up to a hundred and twenty-five, and without the aid of a favorable wind that's about the best we can do. Figuring the way they've been crawling up on us, even with us going at a hundred and fifteen, they must be doing something like a hundred and thirty-five at least."
He pondered for a moment and turned another backward glance. "Well," he ejaculated at last. "I guess those papers are more important than even we guessed. Those fellows aren't coming through on petrol: they're using a mixture of at least twenty-five per cent ether!"
Twenty minutes elapsed and it developed into what well might have been a life-and-death race, with the pursuing plane steadily cutting down the intervening distance—steadily gaining on the one that already was plowing through the air at the rate of a hundred and twenty-seven miles an hour according to the air-speed indicator, and probably not less than a hundred and twenty miles an hour ground speed.
Another half hour and the pursuing machine had sufficiently reduced the distance to let go the first volley of shots from her machine gun.
"Ah," exclaimed Jack, "prepared for action, eh? Well, maybe we can give them a little surprise. I don't think they know we're armed."
He started climbing, and so suddenly and at such an acute angle that the pilot of the other plane could not see the intended maneuver soon enough to parallel the course.