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