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THE POPULAR SCIENCE MONTHLY.

RECENT PROGRESS IN AËRIAL NAVIGATION.
By Professor W. LE CONTE STEVENS.

BALLOONING has thus far been a French art: introduced a little over a century ago by a Frenchman, Montgolfier; rapidly developed by another Frenchman, Charles; more practiced in France than anywhere else in the world; and recently improved by Frenchmen to such an extent that it is quite possible now on any fair day to go an hour's journey through the air in any desired direction, even against the wind.

The history of the application of science to art has revealed a number of cases in which practical success was secured by inventors entertaining quite erroneous conceptions of the principles they were applying. Somewhat vague stories are transmitted in regard to Roger Bacon's suggestion, during the thirteenth century, of employing a thin hollow globe of copper, "to be filled with ethereal air or liquid fire and then launched forth from some elevated point into the atmosphere, where it will float like a vessel on water." Bacon gave no recipe for making "liquid fire," nor did he calculate the dimensions of a globe of copper to be filled with it that would have sufficient ascensive power to lift a human being. He assures us "there is certainly a flying instrument, not that I ever knew a man that had it, but I am particularly acquainted with the ingenious person who contrived it." His conception was never reduced to practice. It was merely a fair specimen of current science in his time. He believed that the aërial ocean around our earth had a definite boundary like the liquid ocean, and that a body of sufficient lightness, if it could only be found, would easily rise to this surface as a cork rises to the surface of water.

More than three centuries after the time of Bacon, Father Lana wrote out his idea of a vessel that might be made to rise in the air. Four hollow globes of copper, each having a diameter of about twenty-five feet, were to be carefully exhausted and then attached to a car. Torricelli and Pascal had already proved that the pressure of the atmosphere was nearly fifteen pounds per square inch at sea-level, and Lana's proposed method of exhausting his globes was to be an application of Torricelli's principle. Each globe was to be filled with water and lifted to a height of at least thirty-four feet. Beneath it should be fitted a tube with air-tight connections, which was to dip into water. On opening this tube the contents of the globe would be emptied into the vessel below, leaving a Torricellian vacuum above, while the tube would become a water-barometer. Lana seemed to know nothing about the specific gravity of gases. His copper globes were to be made very thin in order to secure lightness, but he failed to make any correct estimate of the rigidity they must have to sustain either the