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.