LITMIR.BIZ

      chapter 7

      Geometric details—calculating the balloon’s capacity—the two-piece vehicle—the envelope—the gondola—the mysterious mechanism—provisions—the bottom line.

      Dr. Fergusson was busy for a good while with the details of his expedition. You can appreciate that his balloon, that wondrous conveyance designed to carry him through the clouds, was the recipient of his constant attention.

      First of all, to keep his lighter-than-air vehicle from taking on excessively grand dimensions, he decided to inflate her with hydrogen, a gas that’s 14½ times lighter than oxygen. This gas is easy to produce and has provided the best results in experiments with lighter-than-air vehicles.

      Working with ultracareful calculations, the doctor found that in order to bring the articles essential to his journey, plus his mechanism, he needed to carry aloft a weight of 4,000 pounds; so he had to find out what the lifting power would be that could heft this weight, and consequently, what its capacity would be.

      A weight of 4,000 pounds is equivalent to 44,847 cubic feet¹ of displaced air, which is just another way of saying that 44,847 cubic feet of air weighs about 4,000 pounds.

      When you give the balloon this capacity of 44,847 cubic feet and replace air with hydrogen (a gas that’s 14½ times lighter and weighs only 276 pounds, a difference of 3,724 pounds), you affect the vehicle’s buoyancy. It’s this difference between the gas’s weight inside the envelope and the air’s weight outside that constitutes the balloon’s lifting power.

      Even so, if the balloon took in that 44,847 cubic feet of gas we’re talking about, she would fill up completely; now then, that’s a non-starter, because as the balloon rises into thinner layers of air, the gas inside her tends to expand and will pretty quickly burst the envelope. So as a general rule, balloons are only two-thirds full.

      But due to certain plans known only to him, the doctor decided to fill his lighter-than-air vehicle only halfway, and since he had to carry aloft 44,847 cubic feet of hydrogen, to give his balloon roughly twice the capacity.

He designed her with that stretched-out shape we know to be preferable; the horizontal diameter was 50 feet and the vertical diameter 75;* in this way he got a spheroid with a capacity that amounted to 90,000 cubic feet in round numbers.

      If Dr. Fergusson had been able to use two balloons, it would have increased his chances of success; in essence, if one of them happened to rupture in the air, you could drop some ballast and stay aloft with just the other. But handling two lighter-than-air vehicles turns out to be quite tricky when it comes to giving them the same lifting power.

      After mulling it over a good while, Fergusson devised a clever arrangement that combined the advantages of two balloons without the drawbacks; he built two different-sized envelopes and enclosed one inside the other. His outer balloon, which kept the dimensions we’ve given above, contained a smaller one the same shape with a horizontal diameter of only 45 feet and a vertical diameter of 68 feet. So the capacity of this inner envelope was only 67,000 cubic feet; it would float in the elastic fluid that surrounded it; a valve opened from one balloon to the other, allowing for contact between the two as needed.

      The advantage of this setup was that if you had to let out gas in order to descend, you could release it from the outer balloon first; even if you needed to empty the bigger one completely, the smaller one would stay intact; so you could get rid of the outer envelope as an inconvenient burden, and the second envelope, left on its own, would give the wind less purchase than half-deflated balloons do.

      Furthermore, in the event of a rip or other accident happening to the outer balloon, the inner one had the advantage of being protected.

      This two-part vehicle was manufactured with twill taffeta from Lyon that had been coated with gutta-percha. This is a resinous, rubbery substance that’s completely watertight; it’s also totally impervious to acids and gases. A double layer of taffeta covers the very top of the sphere, which takes almost all the strain.

      This envelope could hold its elastic fluid for an unlimited period of time. It weighed half a pound for every 9 square feet. Now then, the outer balloon had about 11,600 square feet of surface, so its envelope weighed 650 pounds. The second balloon’s envelope had 9,200 square feet of surface and weighed only 510 pounds: so the two came to 1,160 pounds.

      The netting designed to hold up the gondola was made of extra-tough hempen rope; the two valves were looked after as meticulously as if they were a ship’s rudder.

      The wickerwork gondola was circular in shape, 15 feet across, reinforced with a light iron framework, and decked out underneath with elastic springs designed to deaden shocks. Gondola and netting together weighed no more than 280 pounds.

      Beyond this, the doctor had ordered four tanks built from sheet metal a sixth of an inch thick; they were connected to each other by pipes equipped with spigots; attached to them was a coil made of two-inch-wide tubing that ended in two straight, different-sized extensions, the longer measuring 25 feet in height, the shorter just 15 feet.

      The sheet-metal tanks were fitted into the gondola in such a way as to take up the least possible space; the coil didn’t need to be eased in until later and was packed separately, as was a very powerful Bunsen battery. This mechanism had been so cleverly put together, it weighed no more than 700 pounds, even with its special 25-gallon water tank.

      The instruments set aside for the journey consisted of two barometers, two thermometers, two compasses, a sextant, two chronometers, an artificial horizon, and an altazimuth, a telescope for surveying things far away and out of reach. The Greenwich Observatory had put itself at the doctor’s disposal. But Samuel didn’t propose to do any physics experiments; he simply wanted to work out his heading and fix the positions of the chief rivers, mountains, and towns.

      He also took along three well-tested iron anchors, likewise a 50-foot ladder of light, tough silk.

      He calculated as well the exact weight of his provisions; they consisted of tea, coffee, crackers, salted meat, and pemmican, a mixture that contains many nourishing ingredients in concentrated form. In addition to an adequate supply of brandy, he arranged for two water tanks that held

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