AEROSTATION. 



v,hicU is specifically, or bulk for bulk, 

 lighter than the atmospheric air encom- 

 passing tlu- earth, will be buoyed up by 

 it, and ascend; but as the density of the 

 atmosphere decreases, on account of the 

 diminished pressure of the superincum- 

 bent air, and the elastic property which 

 it possesses at different elevations above 

 the earth, this body can rise only to a 

 height, in which the surrounding' air will 

 be of the same specific gravity with itself. 

 In this situation it will either float, or be 

 driven in the direction of the windorcur- 

 rentof air,t whiclv-it is exposed. An air- 

 balloon is a body of this kind, the whole 

 mass of which, including its covering and 

 contents, and the several weights annexed 

 to it, is of less specific gravity than that 

 of the air in which it rises. Heat is well 

 known to rarefy and expand, and conse- 

 quently to lessen the specific gravity of 

 the air to which it is applied; and the di- 

 minution of its weight is proportional to 

 the heat. One degree of heat, according 

 to the scale of Fahrenheit's thermometer, 

 seems to expand the air about one four- 

 hundredth part ; and about 400, or rather 

 435, degrees of heat will just double the 

 bulk of a < (iiantity of air. If, therefore, 

 the air inclosed in any kind of covering 

 be heated, and consequently dilated to 

 such a degree, as that the excess of the 

 weight of an equal bulk of common air, 

 above the weight of the heated air, is 

 greater than the weight of the covering 

 and its appendages, this whole mass will 

 ascend in the atmosphere, till, by the cool- 

 ing and condensation of the included air, 

 or the diminished density of the surround- 

 ing 1 air, it becomes of the same specific 

 gravity with the air in which it floats; and 

 without renewed heat it will gradually 

 descend. If, instead of heating common 

 air inclosed in any covering, and thus di- 

 minishing its weight, the covering be filled 

 with an elastic fluid, lighter than atmo- 

 spheric air, so that the excess of the weight 

 of an equal bulk of the latter, above that 

 of the inclosed elastic fluid, be greater 

 than the weight of the covering and its 

 appendages, the whole mass will, in this 

 case, ascend in the atmosphere, and con- 

 tinue to rise, till it attains aheight at which 

 the surround'ngair is of the same specific 

 gravity with itself. Inflammable air, or, 

 as it is called, hydrogen gas, is a fluid of 

 this kind. For the knowledge of many of 

 its properties we arc indebted to Mr. lien- 

 < ndish, who discovered, that if 

 common air is eight hundred times lighter 

 than water, inflammable air is seven times 

 lighter than common air ; but if common 



air is eight hundred and fifty times light . 

 f-rthanv /ater, then inflammable air is 10.8 

 times lighter !h:ui common air. The con- 

 struction of air-h:i!loons depends upon the 

 principles above stated ; and they are of 

 two kinds, as one or the other of the pre- 

 ceding methods of preparing them is 

 adopted. 



In the various schemes that have been 

 proposed for navigating through the air, 

 some have had recourse to artificial wings, 

 which, being constructed like those of 

 birds, and annexed to the human body, 

 might bear it up, and by their motion, pro- 

 duced either by mechanical springs, or 

 muscular exertion, effect its progress in 

 any direction at pleasure. This is one of 

 the methods of artificial flying suggested 

 by Bishop \Vilkins, in the seventh chapter 

 of his " Dedal us, or Treatise on Mecha- 

 nical Motions;" but the success of it is 

 doubtfid, and experiments made in this 

 way have been few and unsatisfactory. 

 Borelli, havingcomparedthe powerofthe 

 muscles which act on the wings of a bird 

 with that of the muscles of the breast and 

 arms of a man, finds the latter altogether 

 insufficient to produce, by means of any 

 wings, that motion against the air, which 

 is necessary to raise a man in the atmo- 

 sphere. Soon after Mr. Cavendish's dis- 

 covery of the specific gravity of inflam- 

 mable air, itoccurred to the ingenious Dr. 

 Black, of Edinburgh, that if a bladder, 

 sufficiently lightand thin, were filled with 

 this air, it would form a mass lighter than 

 the same bulk of atmospheric air, andrise 

 in it. This thought was suggested in his 

 lectiu'es in 1767 or 1768; and h$ propos- 

 ed, by means of the alantois of a calf, to 

 try the experiment. Overemployments, 

 however, prevented the execution of his 

 design. The possibility of constmcting 

 a vessel, which, when filled with inflam- 

 mable air, would ascend in the atmo- 

 sphere, had occurred also to ^!r. Cavallo, 

 about the same time ; and to him belongs 

 the honour of having first made experi- 

 ments on this subject, in the beginning 

 of the year 1782, of which an account was 

 read to the Royal Society, on the 20th of 

 June in that year. He tried bladders r 

 but the tlu'nnest of these, however, scrap- 

 ed and cleaned, were too heavy. In using 

 China paper, he found that the inflamma- 

 ble air passed through its pores, like wa- 

 ter through a sieve ; and having failed of 

 success by blowing this air into a thick 

 solution of gum, thick varnishes and oil- 

 paint, he was under a necessity of being 

 satisfied with soap-bubbles, which being 

 inflated with inflammable air, by dipping 1 



