Ms Watt said she noted some of what was said on the back of a used envelope on the bus home.

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To travel undetected in the guise of a standard business letter, the device needed to fit into the most commonly used envelope (a number ten in America, which is about 100mm by 240mm).

ECONOMIST: Monitor

They are based on a non-rigid design, in which the pressure of the helium inside the envelope is used to maintain the airship's shape.

ECONOMIST: Pushing the envelope

On at least one occasion, Flowers allegedly used official IRS letterhead and envelope to request more information from a victim, and threatened the victim with an IRS audit if he did not cooperate.

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