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. [Sidenote: To ascertain the Height of the Balloon by a Quadrant.] 310. To compute the Height and Distance of the Balloon, by Means of a _white_ Flag, or other _visible_ Object, suspended from the Car, at a certain Distance below it. Let the Observer take the Altitude of the Car with a Quadrant: and also the Altitude of the Object or Flag. Then by a Case in plain Trigonometry; if the Altitude of the Car be by the Quadrant 59° = HAC, the Altitude of the Object 55° = HAO, and the Length of the Line veered out be 200 Yards, or otherwise = CO. [Illustration] Then the Complement of HAO = AOH = 35°; and the Complement of the Angle HAC = ACH = 31°; and the Supplement of OAC + ACO = AOC = 145°. Then, CAO 4° : CO 200 :: AOC 145° : AC; and Radius : AC :: CAH 59° : CH 1409 Yards, the Height of the Balloon taken at the Time. Next, Radius : AC :: ACH 31° : AH 846 Yards, which is the horizontal Distance of the Place on the Earth from the Observer, over which the Balloon was then suspended. This Method finds the Height truer than the Barometer, and with fewer Circumstances of Confusion.