That was probably what the engineer was waiting for.
Robur made a sign. The propelling screws immediately stopped, and after running for a mile the "Albatross" pulled up motionless.
At a second gesture from Robur the suspensory helices revolved at a speed that can only be compared to that of a siren in acoustical experiments. Their f-r-r-r-r rose nearly an octave in the scale of sound, diminishing gradually in intensity as the air became more rarified, and the machine rose vertically, like a lark singing his song in space.
"Master! Master!" shouted Frycollin. "See that it doesn't break!"
A smile of disdain was Robur's only reply. In a few minutes the "Albatross" had attained the height of 8,700 feet, and extended the range of vision by seventy miles, the barometer having fallen 480 millimeters.
Then the "Albatross" descended. The diminution of the pressure in high altitudes leads to the diminution of oxygen in the air, and consequently in the blood. This has been the cause of several serious accidents which have happened to aeronauts, and Robur saw no reason to run any risk.
The "Albatross" thus returned to the height she seemed to prefer, and her propellers beginning again, drove her off to the southwest.
"Now, sirs, if that is what you wanted you can reply." Then, leaning over the rail, he remained absorbed in contemplation.
When he raised his head the president and secretary of the Weldon Institute stood by his side.
"Engineer Robur," said Uncle Prudent, in vain endeavoring to control himself, "we have nothing to ask about what you seem to believe, but we wish to ask you a question which we think you would do well to answer."
"By what right did you attack us in Philadelphia in Fairmount Park? By what right did you shut us up in that prison? By what right have you brought us against our will on board this flying machine?"
"And by what right, Messieurs Balloonists, did you insult and threaten me in your club in such a way that I am astonished I came out of it alive?"
"To ask is not to answer," said Phil Evans, "and I repeat, by what right?"
"Do you wish to know?"
"If you please."
"Well, by the right of the strongest!"
"That is cynical."
"But it is true."
"And for how long, citizen engineer," asked Uncle Prudent, who was nearly exploding, "for how long do you intend to exercise that right?"
"How can you?" said Robur, ironically, "how can you ask me such a question when you have only to cast down your eyes to enjoy a spectacle unparalleled in the world?"
The "Albatross" was then sweeping across the immense expanse of Lake Ontario. She had just crossed the country so poetically described by Cooper. Then she followed the southern shore and headed for the celebrated river which pours into it the waters of Lake Erie, breaking them to powder in its cataracts.
In an instant a majestic sound, a roar as of the tempest, mounted towards them and, as if a humid fog had been projected into the air, the atmosphere sensibly freshened. Below were the liquid masses. They seemed like an enormous flowing sheet of crystal amid a thousand rainbows due to refraction as it decomposed the solar rays. The sight was sublime.
Before the falls a foot-bridge, stretching like a thread, united one bank to the other. Three miles below was a suspension-bridge, across which a train was crawling from the Canadian to the American bank.
"The falls of Niagara!" exclaimed Phil Evans. And as the exclamation escaped him, Uncle Prudent was doing all could do to admire nothing of these wonders.
A minute afterwards the "Albatross" had crossed the river which separates the United States from Canada, and was flying over the vast territories of the West.
ACROSS THE PRAIRIE
In one, of the cabins of the after-house Uncle Prudent and Phil Evans had found two excellent berths, with clean linen, change of clothes, and traveling-cloaks and rugs. No Atlantic liner could have offered them more comfort. If they did not Sleep soundly it was that they did not wish to do so, or rather that their very real anxiety prevented them.