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 to substitute flapping wings for rotary propellers, as the former can be suspended near the centre of resistance. C. Danilewsky followed him in 1898 and 1899, but without remarkable results. Dupuy de Lôme was the first to estimate in detail the resistances to balloon propulsion, but experiment showed that in the aggregate they were greater than he calculated. Renard and Krebs also found that their computed resistances were largely exceeded, and after revising the results they gave the formula R＝0·01685 D2V2, R being the resistance in kilograms, D the diameter in metres and V the velocity in metres per second. Reduced to British measures, in pounds, feet and miles per hour, R＝0·0006876 D2V2, which is somewhat in excess of the formula computed by Dr William Pole from Dupuy de Lôme’s experiments. The above coefficient applies only to the shape and rigging of the balloon “La France,” and combines all resistances into one equivalent, which is equal to that of a flat plane 18% of the “master section.” This coefficient may perhaps hereafter be reduced by one-half through a better form of hull and car, more like a fish than a spindle, by diminished sections of suspension lines and net, and by placing the propeller at the centre of resistance. To compute the results to be expected from new projects, it will be preferable to estimate the resistances in detail. The following table shows how this was done by Dupuy de Lôme, and the probable corrections which should have been made by him:—

When the resistances have been reduced to the lowest minimum by careful design, the attainable speed must depend upon the efficiency of the propeller and the relative lightness of the motor. The commercial uses of dirigible balloons, however, will be small, as they must remain housed when the wind aloft is brisk. The sizes will be great and costly, the loads small, and the craft frail and short-lived, yet dirigible balloons constitute the obvious type for governments to evolve, until they are superseded by efficient flying machines. (See further, as to the latter, the article .)

The chief danger attending ballooning lies in the descent; for if a strong wind be blowing, the grapnel will sometimes 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. This danger, however, has been much reduced by the use of the “ripping-cord,” which enables a panel to be ripped open and the balloon to be completely deflated in a few seconds, just as it is reaching the earth. 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 increased by the water deposited on it; and if there is no ballast to throw out in compensation, the velocity is sometimes very great. It is also convenient, 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 scattered 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 impossible to liberate the balloon in such a way as to avoid giving it a rotary motion about a vertical axis, which continues 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. 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 exceedingly delicate.

AEROTHERAPEUTICS, the treatment of disease by atmospheric air: a term which of late has come to be used somewhat more loosely to include also pneumotherapeutics, or the treatment of disease by artificially prepared atmospheres. The physical and chemical properties of atmospheric air, under ordinary pressure or under modified pressure, may be therapeutically utilized either on the external surface of the body, on the respiratory surface, or on both surfaces together. Also modifications may be induced in the ventilation of the lungs by general gymnastics or respiratory gymnastics.

The beneficial effects of air under ordinary pressure are now utilized in the open-air treatment of phthisical patients, and the main indications of benefit resulting therefrom are reduction of the fever, improvement of appetite and the induction of sleep.

The air, however, may be modified in composition or in temperature. Inhalation is the most common and successful method of applying it—when modified in composition—to the human body. The methods in use are as follows: (1) Inhalation of gases, as oxygen and nitrous oxide. The dyspnoea and cyanosis of pneumonia, capillary bronchitis, heart failure, &c., are much relieved by the inhalation of oxygen; and nitrous oxide is largely used as an anaesthetic in minor operations; (2) Certain liquids are used as anaesthetics, which volatilize at low temperatures, as chloroform and ether. (3) Mercury and sulphur, both of which require heat for volatilization, are very largely used. In a mercurial or sulphur bath, the patient, enveloped in a sheet, sits on a chair beneath which a spirit lamp is placed to vaporize the drug, the best results being obtained when the atmosphere is surcharged with steam at the same time. The vapour envelops the patient and is absorbed by the skin. This method is extensively used in the treatment of syphilis, and also for scabies and other parasitic affections of the skin. (4) Moist inhalations are rather losing repute in the light of modern