346. The Workmen may begin the Operation at twelve at Night, or at six in the Morning: and the Time previously fixed for the Exhibition, may be eight or ten Hours after the Operation has commenced.
The Necessity of a Current of fresh Water, throu’ a Pipe of at least half Inch Bore, the larger the better, to supply the overflowing Cistern, cannot be too much insisted on: as the Levity of the Gass almost wholly depends upon so trivial a Circumstance, as that of having a plentiful Supply of cold fresh and soft Water.
347. Observation 3. Supposing the Balloon air-tight, near half the Expence is thus saved in the Inflation.
Besides the greater Probability of calm Weather for the Inflation, if completed before X. in the Morning, more Time is given to remedy Accidents, and rectify Mistakes: the Warmth of the Air likewise encreases.
But above all; if an upper Current carry the Balloon to Sea, the Aironaut may, (as before mentioned) drop into the Sea-Breeze, which will waft him safe back till IV. in the Afternoon, or even later.
CHAPTER LXIX.
MENSURATION OF HEIGHTS.
Rules for calculating Heights by Means of the Barometer and Thermometers.
Section 348.RULES for calculating the Height of Mountains, when applied to those elevated Stations in the Atmosphere attainable only by Means of the Balloon, will henceforward become more useful, and be more frequently practised: as the Lives of Aironauts may depend on a Knowledge of their Height above the Earth; which, not being determinable by Sight, in all Weathers, or at all Times, must be referred to the Barometer and Thermometers, they carry up with them.