page fifty-three

The Aerostat

Last night's room was spent on the lawn looking up. This one goes up. A gas balloon cannot be steered: the only thing an aeronaut controls is height, and the only tools are a valve to let gas out and bags of sand to throw over the side. Today's number fills the envelope, stacks the winds in layers that each blow their own way, and names a town where people are waiting for you. Find the heights that take you there, and come down before dark with sand left to land on.

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The country below
your track, low your track, high the subscribers' mile
Time—
Height—
Barometer—
Going—
Wind here—
Sand—
Gas aboard—
Guide rope—
To the subscribers—
Height through the afternoon

Below the blue dashes the envelope is slack and the balloon keeps the same lift at any height, so it will not settle by itself: it goes on rising or sinking until you act. At the dashes it is full, and above them gas spills from the neck and is gone. The green arrow is the wind at your height; the white arrows are the last pilot balloon's.

Day
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On the ground, the arrow keys walk to other days. Every day is a different year, gas, envelope, sky and town.

What is real here and what is not

The way a gas balloon behaves, the gases and the guide rope are real. The towns, the winds and the subscribers are the day's number.

  1. 01

    Steering by height

    Winds often blow different ways at different heights: near the ground friction slows them and turns them to the left of the wind aloft, and above that the layers can go their own way. A balloon goes with the air it is in, so the only way to choose a direction is to choose a height. Aeronauts sent up small paper pilot balloons and watched them through a glass to see what the winds above were doing.

  2. 02

    Slack, then full

    A balloon left the ground partly filled. As it rises the air thins and the gas swells, so the lift stays the same until the envelope is full at its pressure height. Above that, gas is pushed out of the open neck and lost, which is what stops the climb. Below it nothing holds the balloon at a height, which is why aeronauts were always busy with the valve and the sand, and every bit of gas let go had to be paid for in sand later. Evening is hard because the sun stops warming the gas and its lift falls away.

  3. 03

    Hydrogen, coal gas and the rope

    The first hydrogen balloon went up from Paris in 1783, filled from casks of iron and sulphuric acid. In 1821 Charles Green used coal gas from the London mains, which lifts only about two thirds as much but was cheap and to hand, so the envelopes grew. Green also took up the guide rope in the 1820s: a heavy rope hanging below the car, which lies on the ground as the balloon sinks and takes its weight off, so the balloon floats along a little above the fields.

  4. 04

    What the page simplifies

    The air has one temperature and a scale height of 8,000 metres, the winds change smoothly between nine heights and turn slowly through the afternoon, the sun's warming is a single number that fades towards sunset, and the balloon rises and sinks at a speed set by how light or heavy it is. The country is drawn flat with no hills or trees, and the towns are made up.

Fifty-three rooms, and one number doing all the work. Today it fills an envelope, stacks the winds in layers and tells a town to expect you.