150 AERONAUTICS the opening of the valve, and one generally adopted, is to inflate the balloon only partially at the surface of the earth. Mr. Glaisher is Balloon Valve Invented by M. Giflard. of the opinion that in order to reach great al- titudes the balloon most have a capacity of at least 90,000 cubic feet, of which not more than one third need be inflated with gas, and must carry upward of COO pounds of ballast. With such a machine he reached a height of seven miles, at which elevation, as we have seen, he became insensible and his companion nearly so. The question of the extreme altitude to which a balloon can ascend can therefore only be theoretically determined, since the vital powers, however strongly organized, must at 37,000 to 40,000 feet of elevation succumb to the intense cold and the attenuated atmosphere which there prevail. The balloon usually rises in an oblique direction under the com- bined influence of the vertical ascensional force and the direction of the wind. As soon as it mounts into a stratum of air having the same density as itself, it ceases to ascend unless more ballast be thrown out, and follows the course of the aerial current. As regards the particles of air which surround it, it is quite motionless, and the aeronaut may be swept along with the swiftness of a tornado, with nothing to indicate to him, if enveloped in clouds, that he is not in the quiet of a calm. M. Flammarion states that in an aerial journey of 120 m. he never felt himself in motion, and that from a glass of water filled to the brim, which was placed within the car, not a drop was shaken out, although the balloon was con- stantly rising and falling hundreds of feet. Not the least remarkable phenomenon which presents itself to the aeronaut is the concave appearance of the earth, which arches beneath him as the dome of the sky does above, so that he may be said to float between two vast con- cavities. In descending, the aeronaut reduces the buoyancy of the balloon by a skilful man- agement of the rope which controls the safety valve, and when the descent becomes too rapid he lightens the machine by throwing over bal- last. This is an operation which should be committed only to a practised hand. So deli- cately does the balloon respond to any altera- tion in its weight that, as M. Tissandier re- lates, the throwing out of a chicken bone once caused him to rise from 20 to 30 yards. In descending through a heavy bank of clouds the weight of the balloon may also be considerably increased by the deposited moisture, and the most rapid discharge of ballast will sometimes scarcely prevent a violent collision with the earth. Under such circumstances the guide rope suspended from the car, first adopted by Green, proves of great advantage by acting as a sort of substitute for ballast, as every inch of it which rests upon the ground relieves the balloon of an equivalent portion of its weight. Of the innumerable schemes which have been propounded for the guidance and propulsion of balloons, not one has proved available, and the machine is still manageable only for ver- tical motions. It is within the power of the aeronaut to ascend to the utmost height at which human existence is possible, but when he desires to move in a horizontal direction he is for the most part like a rudderless ship at the mercy of the winds and waves. Start- ing from a given point, he may traverse the segment of a circle, or describe the most ec- centric course, and after hours of aerial navi- gation be as far as ever from his proposed goal, lie can rise or fall at pleasure into a current of air seeming to waft him in the desired di- rection ; but so capricious and infinitely vari- ous are the atmospheric streams, and so im- perfectly defined are their courses, that he will be most likely to find himself only baffled and confused by them. In spite of the results which aerostatics offered in connection with the siege of Paris, Mr. Glaisher seriously doubts the practical use of the balloon. He sees no probability that any method of steer- ing it will be invented, and even intimates that this is not necessarily the first step in aerial navigation, and may possibly have no share in the solution of the problem. He would em- ploy it simply as an aerial observatory, whence an infinite variety of phenomena affecting the laws which control the universe can be noted with a precision not attainable on the surface of the earth. Messrs. Fonvielle and Tissan- dier, on the other hand, believe that the guid- ance of balloons has nothing impossible in it, and lay particular stress upon the use to 'be made by the aeronaut of the natural currents of air flowing at various heights in the atmos- phere. But information with regard to these is at present entirely too vague to justify their confidence. Various plans of aerial ships to be propelled and steered by fans, paddles, sails, or other mechanical contrivances, have been projected in Europe and America, all of which, having been designed in ignorance of or in- difference to the most rudimentary atmospher- ic laws, have .proved failures. Under the con- viction that the balloon can never solve the problem of aerial navigation, the "Aeronauti- cal Society " was established in England a few
