Page:The American Cyclopædia (1879) Volume I.djvu/169

 AERONAUTICS 149 of letters, and descended at Evrenx. From that date to the end of the siege, Jan. 28, 1871, 61 others were despatched. Of these, 54 were sent by the post office department, carrying about 2,500,000 letters, which represented a total weight of nearly 10 tons. Most of them also took carrier pigeons intended to bring back news and replies to the outgoing letters. Comparatively few of the pigeons returned to Paris. In order to. adapt the weight of the return document to the capacity of the bird, long messages and letters were reduced by photography to within an area not exceeding one or two square inches on paper of the thin- nest texture. These slips were generally en- closed in a quill, which was fastened to the central tail feather, and when received they were submitted to the microscope and copied. Most of the balloons were under the manage- ment of sailors, whose nautical training, it was supposed, would peculiarly fit them for .navi- gating the air. Several fell into the hands of the enemy, dropping within the hostile lines ; and one, the Washington, which left Paris on Oct. 12, was subjected, while crossing the Prussian outposts at an elevation of 2,500 to 3,000 feet, to so severe a fire that the travel- lers were obliged to ascend rapidly several hundred feet. Some were carried to consider- able distances beyond the French frontier, and the Ville d'0r!6ans was swept into Norway, and came to anchor 600 m. N. of Christiania. Three have never been heard from since they left Paris. To avoid the enemy's fusillade, it was determined in the latter part of November to despatch the balloons at night ; but as no lights were permitted in them by the govern- ment, the subsequent journeys were attended by unusual perils, the aeronauts being unable to determine their rate or direction of travel- ling or their distance from the earth. To this unwise provision was doubtless owing the loss of the balloons above mentioned, and the ec- centric courses which others took. Gambetta, the leader of the provisional government, was one of the first to leave the city by this means of conveyance, in order to take the control of affairs, at' Tours. On the night of Dec. 2 Dr. Janssen departed in the balloon Volta for the purpose of observing the total eclipse of the sun on Dec. 22. He noticed that the balloon fell at sunrise and rose again when the sun was several degrees above the horizon, and ac- counted for this effect by the fact that the en- velope upon receiving the first beams of the sun began to radiate heat into space and be- came rapidly cooler, deriving less heat from the rising sun than it parted with by radiation. This process being finished, it became again susceptible to the sun's rays and reascended. Since the termination of the war it has been announced that the German government have determined to take active steps to effect im- provements in military ballooning, and to make it a part of their system. Notwithstanding nearly a century has elapsed since the inven- tion of the balloon, little or no improvement has been made upon its original form. It con- sists now, as in the time of Montgolfier, of a spheroidal bag of gas enclosed within a net- work, attached by ropes to a ring or hoop, from which is suspended a car for the convey- ance of the aeronaut. In place of heated air The Modern Balloon. or hydrogen, the latter of which is expensive and requires an elaborate apparatus for its production, it has for many years been cus- tomary to use carburetted hydrogen or com- mon coal gas, the mean density of which is about one half that of the air. This improve- ment in aerostatics was first introduced by Mr. Green, the English aeronaut. The height to which a balloon will rise is determined from the law according to which the density of the atmospheric strata diminishes as the distance from the earth is increased. The buoyant force diminishes with the density, and when it is reduced to a quantity only equal to the weight of the balloon and its appendages, no further ascension can take place. As the pressure of the external air is diminished the expansive force of the confined gas becomes greater ; and a balloon quite filled at the sur- face of the earth would inevitably be torn to shreds at the height of a few miles, unless a portion of the confined gas were allowed to es- cape. For this purpose the neck of the bal- loon, into which the gas is introduced, is com- monly left open, and the machine is also fur- nished with a safety valve at the top, which can be opened or shut at pleasure. The valve shown on the next page, invented by M. Giffard, a well known manufacturer of balloons, is considered by M. Tissandier and other high authorities to be perhaps the best thus far made. It consists of a metallic disk four feet in diameter, which is pressed against a wood- en hoop by sixteen steel springs ; by means of the rope attached to its centre it may be held open, but on this being released it springs back to its place. A good precaution, besides