It shoud also be furnished with a Valve, at the Bottom, of equal Diameter with the Trunk: keeping itself Air-tight; and opening outwards by a given Resistance, (as that of ten Pounds Troy,) from the inside Gass.
There must be an upper Valve as usual: occasionally to promote a swift Descent.
308. The Form will likewise continue to be defective, till an interior Balloon for common Air is adopted, according to the Plan laid down by the ingenious Mons. Meunier, lately appointed by the French Academy of Sciences at Paris, one of the Commissioners for the Improvement of Airostation.
The Use of which interior Balloon by Compression of the surrounding Gass in the external Balloon, prevents, it is said, the Loss of Ballast and of Gass: two very considerable Advantages.
For the actual Sum total of Gass not being diminished; the Balloon will continue longer in the Air, before an Escape of Gass, throu’ the Pores of the Silk, makes it descend.
There will, on the same Account, be less Occasion to take in meer Ballast, for the Purpose of throwing it overboard, to prevent the Descent.
Therefore an equal Weight of Articles necessary to remain in the Car, may be substituted in Place of the Ballast.
309. Art. 1. And, since it is next to impossible, the Atmosphere shoud continue for 24 Hours together, of the same Density, Weight, and Temperature; or, in short, without Motion;—the Aironaut will have a Power of seeking, at different Heights, for that Current of Air, or Wind, which suits him best: or, in a very few Minutes, to rise above all Currents; become stationary, and lie to in the serene, waiting for a Wind: which, as before mentioned, he may readily find, by lowering out a Mile of Twine, and his white Flag: attending to it, with a small perspective Glass, or Magnifier.
309. 2. Another most material Advantage is to be able, in a high Wind, to chuse the Spot on which he proposes to alight: or wait for a favourable Opportunity to descend.
To ascertain the Height of the Balloon by a Quadrant.