normal temperature at great heights above the earth, the limit
of ascent for a balloon of the Montgolfier type was estimated by
him at under 9,000 feet. Moreover, since the amount of fuel
that could be carried for maintaining the heat of the balloon
after inflation was subject to definite limits, prescribed by
the carrying capacity of the balloon, the duration of the
journey was necessarily limited just as strictly.

These considerations tended to turn the minds of those
interested in aerostation to consideration of the hydrogen
balloon evolved by Professor Charles. Certain improvements had
been made by Charles since his first construction; he employed
rubber-coated silk in the construction of a balloon of 30 feet
diameter, and provided a net for distributing the pressure
uniformly over the surface of the envelope; this net covered the
top half of the balloon, and from its lower edge dependent ropes
hung to join on a wooden ring, from which the car of the balloon
was suspended--apart from the extension of the net so as to
cover in the whole of the envelope, the spherical balloon of
to-day is virtually identical with that of Charles in its method
of construction. He introduced the valve at the top of the
balloon, by which escape of gas could be controlled, operating
his valve by means of ropes which depended to the car of the
balloon, and he also inserted a tube, of about 7 inches
diameter, at the bottom of the balloon, not only for purposes of
inflation, but also to provide a means of escape for gas in case
of expansion due to atmospheric conditions.

Sulphuric acid and iron filings were used by Charles for filling
his balloon, which required three days and three nights for the