6o6 POPULAR SCIENCE MONTHLY. 



discovery of aerostats was not made till toward the end of the 

 eighteenth century. Lalande was therefore much in the wrong 

 when he said " it was so simple ! why was it not done before ? " 



It would not be just, however, to refer the discovery of aero- 

 stats solely to the efforts of the Montgolfiers. Like all inventors, 

 like Lavoisier himself, these brothers, as Figuier has remarked, had 

 the benefit of a long series of isolated labors, carried on often with- 

 out special purpose, by which the elements of their invention had 

 been gathered up. 



Pere Lana, of Brescia, conceived a plan in 1670 for construct- 

 ing a ship which should sustain itself in the air and move by the 

 aid of sails. Four copper globes, in which a vacuum had been pro- 

 duced in order to render them lighter than the volume of air dis- 

 placed, were to support the ship while the sails propelled it. The 

 scientific conception of the empty globes was correct, but Pere 

 Lana did not think of the enormous collapsing force which the 

 atmospheric pressure would exercise upon them. The idea of a sail 

 which would give his aerial boat a resemblance to a vessel driven 

 by the winds was wholly erroneous. 



Sixty-five years later, in 1735, Pere Galien, of Avignon, gave 

 a fairly clear expression to the theory of aerostats. Resting on 

 the principle of Archimedes, he maintained that if he could fill a 

 globe made of light cloth with a sufficiently rarefied air the globe 

 would necessarily possess an ascensional force, which would permit 

 it to lift itself up in the air with a ship and all its cargo. He 

 proposed to draw this rarefied air from out of the upper regions 

 of the atmosphere, down from the summits of high mountain?, 

 forgetting that the air, when brought down to the level of the 

 ground, would contract in volume and assume the density of the 

 ambient atmosphere. 



In the condition of ignorance of the properties of gases that 

 existed in that age, it did not occur, and could not have occurred, 

 to Pere Galien to use other gases than air; no more could he 

 have thought of employing heat to rarefy the air, for the first not 

 very precise notions on the decrease in densities of gases by heat 

 only date from Priestley. But when Cavendish, in 1765, had 

 fully studied hydrogen gas, and shown that as it was prepared then 

 it was seven times lighter than air. Black was enabled to suggest 

 that by filling a light bag with hydrogen the bag would be able 

 to raise a certain weight in the air. The labors of Cavendish, 

 Black, and the discoveries of oxygen, nitrogen, and other gases by 

 Priestley, were described by Priestley a few years afterward in 

 the celebrated book on The Different Kinds of Air — a book 

 which Stephen and Joseph Montgolfier had in their possession. 



