Galien, a French monk, published a book L'art de naviguer dans l'air in 1757, in which it was conjectured that the air at high levels was lighter than that immediately over the surface of the earth. Galien proposed to bring down the upper layers of air and with them fill a vessel, which by Archimidean principle would rise through the heavier atmosphere. If one went high enough, said Galien, the air would be two thousand times as light as water, and it would be possible to construct an airship, with this light air as lifting factor, which should be 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.