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Rh against this, and by the aid of the rudder to turn the machine into any desired direction. He thus proved incontestably that the problem of directing an aërial steamer was by no means insoluble. A second ascent was accomplished by him in 1855, but under unfavorable conditions. He made no further attempts with this machine, the abandonment of these experiments being due chiefly to their danger. A steam-engine, sending forth sparks beneath a mass of thirty thousand cubic feet of inflammable gas contained within an envelope of thin cloth, is a source of peril to which few men would be willing to expose themselves, even if lofty elevations were not reached. Trouble also resulted from the fact that the weight of the balloon could not be kept constant. The loss of the products of combustion and of spent steam made it difficult to preserve the proper relation between the ascensive power and the weight to be sustained.

Not quite twenty years after Giffard's experiments the problem was again attacked by M. Dupuy de Lôme. His immense balloon, containing one hundred and twenty thousand cubic feet of pure hydrogen, was nearly similar in shape to that of Giffard. The car beneath was capable of carrying easily fourteen men, seven of whom at a time were employed in working a capstan which controlled the shaft of the propeller, each of the two blades of this being about ten feet in length. On February 2, 1872, Dupuy de Lôme ascended in this balloon, and attained a speed estimated at 2·8 metres per second, equivalent to about six miles per hour. By means of the rudder he changed the direction through an angle of 12°. Giffard had attained an estimated speed of four metres per second, or nine miles per hour. Muscular power was thus shown to be far too uneconomical, while steam was too dangerous, to be employed in the direction of aërostats.

It was not until 1881, the year of Giffard's death, that electricity was applied as a motive power in the attempt to solve the difficult problem with which he had grappled. His pupil, M. Gaston Tissandier, had early imbibed a passion for aëronautics, and made many successful ascents with spherical balloons. It was Tissandier who, in company with two friends, ascended in April, 1875, to the height of eight thousand six hundred metres, each of the three swooning on account of the rarefaction of the air, even before this limit was attained. The same aëronaut, in company with Fonvielle, was borne by the wind, in February, 1869, in thirty-five minutes, from Paris to Neuilly-Saint-Front, a distance of fifty miles. Keeping abreast with the progress of electrialelectrical [sic] science, Tissandier conceived the idea of employing storage batteries instead of steam or hand power, as the immediate source of energy to actuate the propeller of an elongated balloon. He constructed a small experimental balloon, which was filled with hydrogen,