From chapter 6 of SIGN, Holmes makes an interesting observation about Small's trail disappearing:
"What the deuce is the matter with the dog?" growled Holmes."They surely would not take a cab, or go off in a balloon."
He says this as if the idea of a balloon came as quickly to his mind as a cab did, and Watson does not seem a bit surprised by his choice of transportation...
Holmes + Watson + hot-air-balloon--you decide.

A/N: Can this be my answer?

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Sherlock Holmes and Doctor Watson were in a hot-air balloon. Suddenly a huge gust of wind blew them far off course. They were quite lost, and looked for someone to give them directions. They spotted a man on the ground and lowered the balloon. "Excuse me, sir," Holmes called, "Can you tell us where we are?"

The man thought for a minute. Then he looked up and said "Gentlemen, you are in a hot-air balloon."

Just then the wind picked up and the balloon was blown high into the air, away from the man on the ground. Holmes turned to Watson. "I deduce, my dear Watson, that that man was a mathematician."

"My goodness, Holmes! How do you know that?"

"For three reasons. First, he thought for a minute before giving us his answer. Second, his answer was absolutely correct. And third, his answer was of no use to us whatsoever!"

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#bow# Thank you, thank you. That's a joke I read on a Sherlock Holmes website years and years ago--I have since lost the site, so I just wrote this down verbatim--you get the gist, anyway. Apologies to any mathematicians--I did not write the joke.

Anyway, on with the story! Er, "balloon" starts with a "B," so it was difficult to resist the 221B-ness radiating from the prompt...

"Absolutely not, Holmes."

"Oh come, Watson—"

"Forget about it."

"We simply need to see if it is possible—Mr. Vigny will be with us the entire time; he's landed balloons in all sorts of dangerous situations."

"I do not wish to find myself in 'all sorts of dangerous situations,' Holmes. Call me a coward if you wish, but I choose to avoid dangerous situations when possible."

"And yet we share rooms."

"So I'm not very good at it. I can still spot an overly dangerous situation when one is presented."

"Watson, it is a perfectly simple matter. We simply need to determine whether or not it is possible to land the thing where Jones says he did."

"Can Vigny not determine this by himself? He's the balloon expert, after all."

"Vigny will be too busy flying to be going about swinging things from the basket. Come, Watson, it will not take long."

"I don't care how long it takes, Holmes, I am not doing it."

"It's hardly dangerous, Watson."

"If I understand you correctly, one of us will be hanging a significant amount out of the basket, though we will be at a significant height."

"Well. Yes."

"No, Holmes."

"Watson—"

"Forget about it, Holmes. Nothing you can say could possibly convince me to go anywhere near that blasted balloon!"

A/N: You know you're in trouble when your author's note is longer than the story itself... #embarassed# And BONUS POINTS to anyone who knows what Vigny's first name is!