428. The true Height, in Feet and Tenths, of the Barometer in the Car 6998.2 Feet in a Yard 3) —————— Yards in a Mile 1760) 2332.2 Feet. 1760 (1 Mile. —————— Yards in a Quarter of a Mile 440 ) 572 (1 Qr. 440 ——— 32 Yards. The Height of the Balloon 1 Mile, 1 Quarter, 32 Yards, and 2 Feet. _End of the last Stage, and of the Mensuration of Heights._ N. B. A _thermometric_ sliding Rule, for the Expansion of Quicksilver, and of Air, may possibly, from the foregoing Tables, be so contrived and adapted to the Barometer, as to tell the Height by Inspection, while in the Car of the Balloon. CHAPTER LXXX. HINTS, ON THE CHEAPEST METHOD OF INFLATING BALLOONS, WITH DESCRIPTIONS OF DIFFERENT MODELS FOR A GASS-STEAM-ENGINE. Section 429. The _Expence_ attending the Inflation of Balloons is a solid Objection to their frequent Use. A Check is thereby given to every Improvement that might otherwise be expected from a Repetition of Experiments. It is, in short, the chief Difficulty under which the +aironautic art+ at present labours. This Difficulty, however, if once overcome, (and of which there is little Doubt) will probably bring those extraordinary Machines, into general Estimation. What _now_ costs fifty Pounds, may _then_ be done for five: abating the Expence of the preparatory Engine. Mons. Lavoisier, by the Application of Steam to Iron Filings enclosed in a Copper Retort, has generated inflammable Air, or light Gass:[136] and Dr. Priestley, by converting a Gun-Barrel into a Steam-Engine, has produced a Gass 13 Times lighter than common Air;[137] whereas by the present expensive Method, with Metal and Acid, the Gass for Inflation is seldom more than six Times lighter. What has hitherto been atchieved on a small Scale, is here meant to be extended. As no Particulars are made public, or at least, have yet come to the Author’s Knowledge, relative to the Construction of such a Gass-Steam-Engine, as may, with Safety and Effect, be applied to the Inflation of Balloons; the following Descriptions of different Models may deserve some Notice:—may possibly excite the Attention of the ingenious; and put them on contriving _easier_ Means to obtain the _same_ End. I.