Today, human energy. The University of Houston's
College of Engineering presents this series about
the machines that make our civilization run, and
the people whose ingenuity created them.
The walk to the top of New
Hampshire's Mount Washington, the highest point in
the northeastern United States, is about four
miles. But you also increase your elevation by over
half a mile. Suppose that you, your clothing, and
your pack all weigh 180 pounds. You'll have done
about 500,000 foot-pounds of work lifting yourself
up the mountain. You'll almost certainly be quite
tired.
You burn up part of your own body doing that, but
less than you might hope. If our bodies were
perfectly efficient, that climb would cost only
around 170 calories. Actually, our bodies are only
twenty-five percent efficient, so it'd take more
like seven hundred calories. Even then,
one hearty meal would put us back where we started.
Well, at least the view from the top is gorgeous.
Still the payoff in exercise is greater than just
the effort of lifting ourselves to the top. We
dissipate around three hundred calories per hour in
straight walking, depending on our weight --
lifting and dropping our legs, swinging our arms,
breathing.
So we have to distinguish effort that simply
dissipates energy from effort that places energy
where we might use it. To make use of it, imagine a
fairly fanciful rig: Suppose, once at the top, we
fit a skateboard with wheels so we can ride back
down the mountain on the cog
railway tracks. Suppose we wrap a long cord
around a drum at the top. As we ride down, the drum
spins and powers a generator. If we travel at
eighteen miles an hour, we can keep seven bright
light bulbs lit until we reach the bottom ten
minutes later. Then our efforts will've been put to
brief use.
Any energy conversion introduces inefficiency, and
human bodies are puny engines. In the nineteenth
century, the British introduced the treadmill to punish prisoners. For
hours they climbed stair steps on a rotating drum
that drove a mill -- climbing four times the height
of Mount Washington each day. Treadmills didn't
catch on in the United States. Humans couldn't
replace water wheels, and we wanted labor for more
specific jobs. Why squander an effort that could be
used to build roads or pick cotton?
A nineteenth-century table of the rates of doing
useful work shows that a man working a pump all
day, or pushing a capstan, produces only a
twentieth of a horsepower. That same man climbing
Mount Washington in two hours might exert an eighth
of a horsepower in lifting himself up. If he takes
it easy and does it in four hours, that drops to
only a sixteenth of a horsepower, and so on.
By 1910 we find new kinds of tables. Instead of
talking about human output, they show how much
machine-made power various industries consume per
employee. Humans are now managers of power.
The printing industry used a third of a horsepower
per employee, while the steel industry used five.
And those numbers continue to rise. You and I would
do better with the problems of energy consumption
if we had a clearer sense of how it all relates to
our own body -- to the one
engine we all know in common.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
For more on energy in its various forms and power,
see any elementary physics or thermodynamics
textbook. See also the various links in the text.
A view from the top of Mount Washington
The Engines of Our Ingenuity is
Copyright © 1988-2003 by John H.
Lienhard.
