Today, let us blow hot and cold. 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.
An old children's story tells
of a traveler lost in a vast forest one winter's
night. He stumbles into a widow's hut and begs for
a bowl of soup by her fire. The woman says, "Yes."
He stands blowing on his hand while she ladles the
soup. "What are you doing?" she asks. "Why, my
hands are cold. I'm warming them with my breath."
She eyes him suspiciously as she hands him his
soup. He sits down with the bowl, and blows across
the spoon before he puts it in his mouth.
"Now what are you doing," she cries. He glances up,
surprised, and says, politely, "The soup is so
wonderfully hot. I simply mean to cool it before I
try to swallow it." The woman seizes a fire-iron
and shouts, "Get out! Get out of my house! I'll
have no sorcerer who can blow both hot and cold
under my roof!"
Interesting story. I suppose it was meant to tell
children to be consistent. And that's something we
all need to think about. But let's look first at
the literal act of blowing hot and cold.
Air leaves our body at a little over 98 degrees
Fahrenheit. When we come in out of the cold, we
open our mouth wide and exhale that warm air upon
our hands. We clear the air passage so the warm air
leaves almost unimpeded.
Cooling soup is another matter. We purse our lips
to raise the pressure of the air in our mouth. The
temperature of exiting air drops and the air accelerates.
That fast-moving air draws the surrounding
room temperature are into it. The resulting jet
is therefore close to room temperature when it passes
over the much hotter soup.
So there is no sorcery. We all really do blow both
hot and cold, and we do it instinctively. Yet
there's that story. And another one like it tells
of a king who grew frustrated with advisors who
kept telling him, "On the other hand..." Finally he
shouted to his chamberlain, "Go out and find me a
one-armed advisor."
Something there is that does not like ambiguity.
Yet reality shifts under our feet. New technology
constantly recreates expectation by making a
different place of our world. So what about blowing
hot and cold? The Bible tells us that, "Because you
are lukewarm
-- neither hot nor cold -- I will spew you from my
mouth."
Maybe the trick is not to blow consistently hot or
cold, but to avoid being maneuvered by uncertainty
into blowing neither hot nor cold. A
Wright-Brothers biographer tells how Orville and
Wilbur would argue so ferociously that you had to
listen closely to hear that they often
exchanged positions during their verbal
combat.
Maybe it's useful to dislike uncertainty. For only
by embracing an idea fully can we really test and
reject it -- as the Wright Brothers did. Perhaps,
if I can paraphrase Barry Goldwater, some extremism
-- some heat and cold -- in the pursuit of truth
may not be entirely a vice.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
For the analysis of the adiabatic decompression of
air, see W. C. Reynolds and H. C. Perkins,
Thermodynamics. New York: McGraw-Hill Book
Co., 1977. See especially p. 249, eqn. 8.33.
Of the two factors, the cooling of the air as it expands from
our mouth, and the entrainment of room temperature air,
the dominant effect is the latter. (I am grateful
to Stein Kuiper, a Senior Scientist at Phillips Research Labs,
Eindhoven, The Netherlands, for pointing out to me
the dominance of room air entrainment in the coolong of soup.)
