430. Let there be an Iron _Hot-hearth_, one Yard square, and two Inches thick. Let it be _set_ on a Common Brick Stove, built as near the Ground as possible, (or even below it) in the open Air. Its Chimney to consist of malleable Iron, flat at the Top, and strong enough to support a Tea-Kettle or Boiler to produce Steam: and extending at least one Yard from the End of the Hearth horizontally, before it turns up. It may rise three or four Yards high, slanting farther from the Hearth: the Form a hollow Cylinder: with a Turn-Cap at the Top, two Feet long, set on at right Angles; for the Management of the Smoke. Supposing then the Fire-Place to face the West; the Chimney may project Eastward. The North Side is to be appropriated to the Iron-Borings or Turnings; and on the South Side is to be deposited the Dross or Calx. A Muffle or Mould of malleable Iron is to be screwed and luted over the hot Hearth. The four Sides of the Muffle next the Hearth are to have horizontal Lips or Rims projecting half an Inch: and Screws are to be driven, throu’ Holes drilled at proper Distances, into the Hearth. The Sides are to rise upright a Couple of Inches: closing, as they rise, in the Form of a hollow Cylinder, one Foot in Diameter, and perhaps a Yard above the Hearth: which is now converted into a _Gass-Steam-Engine_. It is proposed to strew over the _Hot-hearth_ a thin Layer of Borings, one Tenth of an Inch thick; to which Layer when _red_ hot, the boiling Steam is to be applied. The extricated Gass is to be conveyed from the Top of the Cylinder, by Means of an extended Trunk of Tin, and varnished Linen, into a Tub of cold Water kept _continually_ flowing over, into which a few Lumps of quick Lime are thrown: and from thence the Gass is to rise into the Balloon. 431. The Iron, whether Filings or Turnings, proper for Inflation, must be _bright_; wholly free from Chips, Bits of Wood, and all heterogeneous Particles: but particularly RUST, and GREASE: _less_ than a cubic Inch of the latter, woud spoil a Ton of the brightest, and otherwise the best prepared Materials. (Section 339.) A Day or two _only_, before a Balloon is inflated; the proper Quantity of bright Iron shoud be heated +red hot+ in _Charcoal_, and suffered to go cold. For Want of this simple Preparation of the Iron, the Gass has proved defective in Point of _LEVITY_: altho’ the Balloon appeared fully inflated. This Misfortune happened at Birmingham, and other Places. 432. The _Desideratum_ is, _quickly_ to apply, and _remove_ the Borings, keeping the Machine _nearly_ Air-tight. For, it is now well known, that the Gass will _explode_, if one-third Part of common Air be introduced: or, if less; it may _unite_ with the _Gass_, and detract from _its_ Levity. 433. The following _Particulars_ may likewise be considered as an Improvement. II.