Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/225

Rh 207 shape resembles rather that of a pear. The pattern gore should originally be made as if for a spherical balloon, and afterwards&quot; the slight modification necessary for the forma tion of the neck should be applied. itruc- The gores are sewn together, and a small portion of the of a upper end of each is cut away, so as to leave an aperture at )on - the top of the balloon of from 1 to 3 feet in diameter. This space is occupied by the valve, which is generally made of strong wood, and consists of two semicircular shutters hinged to a diameter of the circular frame, and kept closed by a spring. The valve is opened by pulling a string, technically called the valve-line, which passes down through the balloon and out at the lower orifice in which the neck terminates. The net-work which, like the gores, is attached to the circumferences of the valve, passes over the surface of the balloon, and supports the ring or hoop from which the car is suspended by half a dozen strong ropes, of perhaps 4 or 5 feet in length. The net work is thus stretched between the valve and the ring. It is very important that all the ropes by which the car hangs from the ring should be so adjusted that each may bear pretty nearly the same weight, as otherwise the whole net ting and balloon will be strained, and perhaps to a serious extent. The car is usually merely a large basket made of wicker-work. The neck of the balloon should be 7 or 8 feet above the car, so that the aeronaut can easily reach it by mounting into the ring. The best material for the envelope is silk; but on account of the expense cotton or alpaca is generally used : in all cases it must be varnished, in order to render it more impervious to the gas. The grapnel or anchor is a large five-pronged hook attached to the ring by a rope 100 or 120 feet long. The first care of the aeronaut on leaving the earth is to lower the grapnel gently to the full extent that the rope will permit. Thus, when the balloon is in the air, the grapnel hangs down below it, and when the descent is being effected, is the first thing to touch the ground. If the descent is well managed, and the balloon is moving downwards slowly, the weight of which it is relieved when the grapnel is supported by the earth checks any further descent, and the wind carries the balloon along horizontally, the grapnel trailing over the ground until it catches in some obstruction and is held fast. The balloon is then in much about the same position as a kite held by a string, and if the wind be strong, plunges about wildly, striking the ground and rebounding, until the aeronaut, by continued use of the valve-line, has allowed sufficient gas to escape to deprive it of all buoyancy and prevent its rising again. otice of The chief danger attending ballooning lies in the descent; ista- f or if a strong wind be blowing, the grapnel will some times trail for miles over the ground at the rate of ten or twenty miles an hour, catching now and then in hedges, ditches, roots of trees, &c. ; and, after giving the balloon a terrible jerk, breaking loose again, till at length some obstruction, such as the wooded bank of a stream, affords a firm hold. If the balloon has lost all its buoyant power by the escape of the gas, the car also drags over the ground. But even a very rough descent is usually not productive of any very serious consequences ; as, although the occupants of the car generally receive many bruises, and are perhaps cut by the ropes, it rarely happens that anything worse occurs. On a day when the wind is light (supposing that there is no want of ballast) nothing can be easier than the descent, and the aeronaut can decide several miles off on the field in which he will alight. It is very important to have a good supply of ballast, so as to be able to check the rapidity of the descent, as in passing downwards through a wet cloud the weight of the balloon is enormously in creased by the water deposited on it; and it there is no ballast to throw out to compensate this accession of mass, the velocity is sometimes very great. It is also convenieTit, if the district upon which the balloon is descending appear unsuitable for landing, to be able to rise again. The ballast consists of fine baked sand, which becomes so scat tered as to be inappreciable before it has fallen far below the balloon. It is taken up in bags containing about cwt. each. The balloon at starting is liberated by a spring catch which the aeronaut releases, and the ballast should be so adjusted that there is nearly equilibrium before leaving, else the rapidity of ascent is too great, and has to be checked by parting with gas. It is almost im possible to liberate the balloon in such a way as to avoid giving it a rotary motion about a vertical axis, which con tinues during the whole time it is in the air. This rotation makes it difficult for those in the car to discover in what direction they are moving; and it is only by looking down along the rope to which the grapnel is suspended that the motion of the balloon over the country below can be traced. We may mention that the upward and downward motion at any instant is at once known by merely dropping over the side of the car a small piece of paper: if the paper ascends or remains on the same level or stationary, the balloon is descending; while, if it descends, the balloon is ascending. This test is so delicate that it sometimes showed the motion at a particular instant with more pre cision than did Mr Glaisher s very delicate instruments. Contrivances are often proposed by which the valve might be opened in less crude ways than by merely pulling a string attached to it; by which the jerks produced by the catching of the grapnel might be diminished, &c. These im provements are not adopted, because simplicity is requisite before everything. Any mechanical contrivance might be broken and rendered useless by the first blow of the car on the earth; whereas the primitive arrangements in use are such that scarcely any rough treatment can impair their efficiency. The most important works that have appeared on tho Works o subject of aerostation are aerosta- Dccdalus, or Mechanical Motions, by Bishop Wilkins, London, 1648 ; A Treatise on the Nature and Properties of Air and other Permanently Elastic Fluids, by Tiberius Cavallo, London, 1781 ; Account of the First Aerial Voyage in England, in a Scries of Letters to his Guardian, by Vincent Lunardi, London, 1784 ; History and Practice of Aerostation, by Tiberius Cavallo, London, 1785 ; Annals of some Remarkable Aerial and Alpine Voyages, including those of the author, by T. Forster, London, 1832 ; Acronautica, by Monck Mason, London, 1838 ; A System of Aero nautics, comprehending its Earliest Investigations, by John Wise, Philadelphia, 1850 ; Astra Castra, Experiments and Adventures in the Atmosphere, by Hatton Turner, London, 1865 ; Voyages Aericns, par J. Glaisher, C. Flammarion, W. de Fonvielle, et G. Tissandier, Paris, 1870 ; the same translated into English and published, edited by James Glaisher, under the title, Travels in the Air, London, 1871. All the above books we have seen ourselves, and used in the preparation of the present article. Astra Castra is a work of 530 pp. large quarto ; it consists chiefly of extracts from other works and writings, and it is useful as affording data for a history rather than as a history itself. On pp. 463-4G5 is a list of books and papers on aeronautics, which seems fairly complete np to the date 1864. In the list are also included memoirs and papers which we have not noted in the last paragraph, as the most important of them are referred to under their special sub jects in the course of this article. We should advise any one desirous of studying the history of aeronautics to consult Mr Tumor s list in Astra Castra, which is the most perfect we have met with. He has marked with an asterisk those works that may be consulted by the public in the library of the Patent Office, which contains, besides books, a valuable collection of prints and broadsheets on the subject of aerostation. (J. G.) tion.