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No. 1796:
Blow Hot; Blow Cold

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: Perhaps it was meant to tell children to be consistent -- something we need to think about. But, before we do, let's look 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. This time we purse our lips and build up air pressure in our mouth. Our lips are now a nozzle through which air leaves at substantial speed. That air jet is now a bit cooler than it was in our mouth, but another cooling effect is dominant. The jet entrains a great deal of cool room air, which flows over the soup convectively cooling it.

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 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."

Does that mean we should all run around blowing hot and cold? I doubt it. Rather, we cannot let timidity drive us to blow neither hot nor cold. A Wright Brothers biographer tells how Orville and Wilbur argued so ferociously that you had to watch closely -- because 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, may not be entirely a vice. Well, as long as it's part of our ongoing pursuit of truth.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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A huge amount of room air (or water in the case of a water jet) is entrained into the jet.  That is the primary reason one can "blow cold."

The secondary effect is air cooling by adiabatic depressurization as it leaves the mouth, see, e.g., W. C. Reynolds and H. C. Perkins, Thermodynamics. (New York: McGraw-Hill Book Co., 1977): especially, pg. 249, eqn. 8.33. The temperature of exiting air is Te = Ti(pe/pi)(k-1)/k, where i denotes conditions outside the mouth, e denotes conditions within the mouth, T is the absolute temperature, and p is the absolute pressure. The constant k is the adiabatic constant, approximately 1.41 for air. This means that the absolute temperature varies as p0.29. The slight pressure difference can change the air temperature by only a few degrees.

T.D. Crouch, Why Wilbur and Orville? Some Thoughts on the Wright Brothers and the Process of Invention, Inventive Minds. R. J. Weber and D. N. Perkins, eds. ( New York: Oxford University Press, 1992): pp. 80-92.

The New Testament passage is from Revelations, 3:16

Snowy woods