Joe said with a laugh. “You’re talking like this because he isn’t here; but when he says to your face: ‘Dick!’ (with all due respect), ‘Dick, I need to know exactly what you weigh,’ I guarantee you’ll go.”
“I won’t go.”
At that moment the doctor reentered his study, where this conversation was taking place; he turned to Kennedy, who wasn’t feeling fully at ease just then.
“Dick,” the doctor said, “come along with Joe; I need to know what the two of you weigh.”
“But—”
“You can keep your hat on. Come along.”
And Kennedy did.
The three of them made their way to the Mitchells’ workshop, where one of those scales called a steelyard balance had been set up. The doctor absolutely had to know his companions’ weight in order to determine the buoyancy of his lighter-than-air vehicle. So he made Dick climb onto the platform of the scale; the hunter didn’t put up any resistance, saying under his breath:
“Fine! Fine! Doesn’t mean I’m agreeing to it.”
“A hundred and fifty-three pounds,” the doctor said, scribbling this number in his notebook.
“Am I too heavy?”
“Naw, Mr. Kennedy,” Joe shot back. “Anyhow I’m on the light side, so I’ll make up for it.”
With that Joe took the hunter’s place, and so enthusiastically that his momentum nearly tipped the scale over; he mimicked the statue of Achilles that honors Wellington at the Hyde Park entrance—and he looked impressive even without a shield.
“A hundred and twenty pounds,” scribbled the doctor.
“Hoho!” Joe threw in while grinning smugly. Why the grin? He wasn’t saying.
“Now my turn,” Fergusson said. And he scribbled 135 pounds for himself.
“The three of us,” he said, “weigh no more than 400 pounds.”
“But master,” Joe continued, “if your experiment called for it, I could easily lose another twenty pounds by skipping meals.”
“No need, my boy,” the doctor replied. “Eat all the meals you like; here’s a half-crown, take on as much ballast as you want.”
* Suburb south of London.
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
