Another project, which sounded logical enough, was based on the irrefutable truth that as air has some weight – to be exact 14.70 pounds for a column one inch square and the height of the earth's atmosphere – a vacuum must be lighter, as it contains nothing, not even air. Accordingly in the seventeenth century, one Francisco Lana, another priest, proposed to build an airship supported by four globes of copper, very thin and light, from which all the air had been pumped. The globes were to be twenty feet in diameter, and were estimated to have a lifting force of 2650 pounds. The weight of the copper shells was put at 1030 pounds, leaving a margin of possible weight for the car and its contents of 1620 pounds. It seemed at first glance a perfectly reasonable and logical plan. Unhappily one factor in the problem had been ignored. The atmospheric pressure on each of the globes would be about 1800 tons. Something more than a thin copper shell would be needed to resist this crushing force and an adequate increase in the strength of the shells would so enhance their weight as to destroy their lifting power.
To tell at length the stories of attempt and failure of the earliest dabblers in aeronautics would be unprofitable and uninteresting. Not until the eighteenth century did the experimenters with lighter-than-air devices show any practical results. Not until the twentieth century did the advocates of the heavier-than-air machines show the value of their fundamental idea. The former had to discover a gaseous substance actually lighter, and much lighter, than the surrounding atmosphere before they could make headway. The latter were compelled to abandon wholly the effort to imitate the flapping of a bird's wings, and study rather the method by which the bird adjusts the surface of its wings to the wind and soars without apparent effort, before they could show the world any promising results.
Nearly every step forward in applied science is accomplished because of the observation by some thoughtful mind of some common phenomenon of nature, and the later application of those observations to some useful purpose.
It seems a far cry from an ancient Greek philosopher reposing peacefully in his bath to a modern Zeppelin, but the connection is direct. Every schoolboy knows the story of the sudden dash of Archimedes, stark and dripping from his tub, with the triumphant cry of "Eureka!" – "I have found it!" What he had found was the rule which governed the partial flotation of his body in water. Most of us observe it, but the philosophical mind alone inquired "Why?" Archimedes' answer was this rule which has become a fundamental of physics: "A body plunged into a fluid is subjected by this fluid to a pressure from below to above equal to the weight of the fluid displaced by the body." A balloon is plunged in the air – a fluid. If it is filled with air there is no upward pressure from below, but if it is filled with a gas lighter than air there is a pressure upward equal to the difference between the weight of that gas and that of an equal quantity of air. Upon that fact rests the whole theory and practice of ballooning.
The illustration of James Watt watching the steam rattle the cover of a teapot and from it getting the rudimentary idea of the steam engine is another case in point. Sometimes however the application of the hints of nature to the needs of man is rather ludicrously indirect. Charles Lamb gravely averred that because an early Chinaman discovered that the flesh of a pet pig, accidentally roasted in the destruction by fire of his owner's house, proved delicious to the palate, the Chinese for years made a practice of burning down their houses to get roast pig with "crackling." Early experimenters in aviation observed that birds flapped their wings and flew. Accordingly they believed that man to fly must have wings and flap them likewise. Not for hundreds of years did they observe that most birds flapped their wings only to get headway, or altitude, thereafter soaring to great heights and distances merely by adjusting the angle of their wings to the various currents of air they encountered.
In a similar way the earliest experimenters with balloons observed that smoke always ascended. "Let us fill a light envelope with smoke," said they, "and it will rise into the air bearing a burden with it." All of which was true enough, and some of the first balloonists cast upon their fires substances like sulphur and pitch in order to produce a thicker smoke, which they believed had greater lifting power than ordinary hot air.
In the race for actual accomplishment the balloonists, the advocates of lighter-than-air machines, took the lead at first. It is customary and reasonable to discard as fanciful the various devices and theories put forward by the experimenters in the Middle Ages and fix the beginning of practical aeronautical devices with the invention of hot-air balloons by the Montgolfiers, of Paris, in 1783.
The Montgolfier brothers, Joseph and Jacques, were paper-makers of Paris. The family had long been famous for its development of the paper trade, and the many ingenious uses to which they put its staple. Just as the tanners of the fabled town in the Middle Ages thought there was "nothing like leather" with which to build its walls and gates, thereby giving a useful phrase to literature, so the Montgolfiers thought of everything in terms of paper. Sitting by their big open fireplace one night, so runs the story, they noticed the smoke rushing up the chimney. "Why not fill a big paper bag with smoke and make it lift objects into the air?" cried one. The experiment was tried next day with a small bag and proved a complete success. A neighbouring housewife looked in, and saw the bag bumping about the ceiling, but rapidly losing its buoyancy as the smoke escaped.
"Why not fasten a pan below the mouth of the bag," said she, "and put your fire in that? Its weight will keep the bag upright, and when it rises will carry the smoke and the pan up with it."
Acting upon the hint the brothers fixed up a small bag which sailed up into the air beyond recapture. After various experiments a bag of mixed paper and linen thirty-five feet in diameter was inflated and released. It soared to a height of six thousand feet, and drifted before the wind a mile or more before descending. The ascent took place at Avonay, the home at the time of the Montgolfiers, and as every sort of publicity was given in advance, a huge assemblage including many officials of high estate gathered to witness it. A roaring fire was built in a pit over the mouth of which eight men held the great sack, which rolled, and beat about before the wind as it filled and took the form of a huge ball. The crowd was unbelieving and cynical, inclined to scoff at the idea that mere smoke would carry so huge a construction up into the sky. But when the signal was given to cast off, the balloon rose with a swiftness and majesty that at first struck the crowd dumb, then moved it to cheers of amazement and admiration. It went up six thousand feet and the Montgolfiers were at once elevated to almost an equal height of fame. The crowd which watched the experiment was wild with enthusiasm; the Montgolfiers elated with the first considerable victory over the force of gravity. They had demonstrated a principle and made their names immortal. What remained was to develop that principle and apply it to practical ends. That development, however, proceeded for something more than a century before anything like a practical airship was constructed.
But for the moment the attack on the forces which had kept the air virgin territory to man was not allowed to lag. In Paris public subscriptions were opened to defray the cost of a new and greater balloon. By this time it was known that hydrogen gas, or "inflammable air" as it was then called, was lighter than air. But its manufacture was then expensive and public aid was needed for the new experiment which would call at the outset for a thousand pounds of iron filings and 498 pounds of sulphuric acid wherewith to manufacture the gas.
The first experiment had been made in the provinces. This one was set for Paris, and in an era when the French capital was intellectually more alert, more eager for novelty, more interested in the advancement of physical science and in new inventions than ever in its long history of hospitality to the new idea. They began to fill the bag August 23, 1783 in the Place des Victoires, but the populace so thronged that square that two days later it was moved half filled to Paris's most historic point, the Champ de Mars. The transfer was made at midnight through the narrow dark streets of mediæval Paris. Eyewitnesses have left descriptions of the scene. Torch-bearers lighted on its way the cortège the central