"I know," agreed Linton, "and it can travel at a high rate of velocity with less expenditure of power." "Exactly; a well-made screw propeller obtains sufficient grip on the air to propel an air-boat at almost any speed; the greater the speed the greater the efficiency of the screw. We are going slowly at this moment, but I could put her along at 70 miles an hour, if one wanted to." Suiting the action to the word, he did increase the speed very considerably for a short distance, and conversation had to be suspended. It was the quickest travelling Linton had yet experienced in the upper air, and he turned with some anxiety to Zenobia Jardine, thinking the pace might tax her nerves. She was perfectly calm, however, and her father set all fears at rest by saying, as he slackened pace again: "The steering with the new gyroscope is almost automatic, just as if she were a torpedo. Even in a stiff wind she reverts to a horizontal keel. It is simply like the balancing of a bird." "The _Bladud_ is splendid!" cried Linton with conviction. "She's hard to beat," was the President's comment. "But, after all, she's only the natural outcome of the air-gun, which has been known for generations. An air-gun is shaped like a rifle, with a hollow boiler or reservoir of power. You force into the reservoir by means of a condensing syringe as much air-power as it will hold. By opening a valve a portion of the air escapes into the barrel of the gun. That's what takes place when you pull the trigger. The released air presses against the ball just as gunpowder would. Off goes your bullet without a sound or sign to show that it has been discharged. Air condensed to 1-46th of its bulk gives about half the velocity of gunpowder. It's precisely the same principle that's firing us through the air at the present moment." "It's a wonderful discovery!" was Linton's comment. "Yes," mused Mr. Jardine, "and yet the thing was always there to be discovered." "Just as the air waves were always ready for wireless telegraphy, but unused till Marconi came along at the beginning of the present century." The President looked around him at the star-spangled heavens and drew in a deep breath: "Yes," he said, slowly, "and there are more secrets waiting to be revealed." "There's a professor of chemistry in one of the American universities who thinks we shall be able to live on air some day," laughed the young man. The President did not laugh. "Why not?" he asked. "We know well enough we can't live without it. It's quite conceivable that the atmosphere contains undetected sources of nourishment. They may be generated by vaporisation or by electricity and chemical action within the air itself. No one knew anything about ozone a hundred and fifty years ago, and he would be a rash man who said that ozone is the last word in atmospheric discovery." "It may end in air cakes," suggested Linton, rather flippantly.