as large as the town of Avignon, and carry four million
passengers with their baggage. How this high air was to be
obtained is matter for conjecture--Galien seems to have thought
in a vicious circle, in which the vessel that must rise to
obtain the light air must first be filled with it in order to
rise.

Cavendish's discovery of hydrogen in 1776 set men thinking, and
soon a certain Doctor Black was suggesting that vessels might be
filled with hydrogen, in order that they might rise in the air.
Black, however, did not get beyond suggestion; it was Leo
Cavallo who first made experiments with hydrogen, beginning with
filling soap bubbles, and passing on to bladders and special
paper bags. In these latter the gas escaped, and Cavallo was
about to try goldbeaters' skin at the time that the Montgolfiers
came into the field with their hot air balloon.

Joseph and Stephen Montgolfier, sons of a wealthy French paper
manufacturer, carried out many experiments in physics, and
Joseph interested himself in the study of aeronautics some time
before the first balloon was constructed by the brothers--he is
said to have made a parachute descent from the roof of his house
as early as 1771, but of this there is no proof. Galien's idea,
together with study of the movement of clouds, gave Joseph some
hope of achieving aerostation through Galien's schemes, and the
first experiments were made by passing steam into a receiver,
which, of course, tended to rise--but the rapid condensation of
the steam prevented the receiver from more than threatening
ascent. The experiments were continued with smoke, which
produced only a slightly better effect, and, moreover, the paper