Today, thoughts about a conflict that scholars
face. 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 eminent engineer once
said to me, "John, our field suffers because we
forget primary sources. We only read papers that
were written the day before yesterday."
The importance of that remark came home to me a few
years ago when my son John began studying fluid
mechanics. I suggested he might try to solve an old
problem that no one had figured out how to do. It
When water flows through a round hole in a plate,
two things happen: The jet contracts to a diameter
smaller than the hole. And water viscosity slows
the jet. Both effects reduce the outflow. Textbooks
say the jet slows about two percent, but no one had
ever predicted the slowing mathematically.
So John made the prediction. The trouble was, he
showed viscosity has almost no effect on jet speed.
He contradicted what everyone thought they knew.
Something was wrong, so he went looking.
The trail led back to a 1940 paper. The authors had
carefully measured the net reduction in flow out of
a hole. But they'd lumped contraction and slowing
They referred to measurements in an old paper,
written in 1906. They found a flaw in the old
contraction measurements. After that, no one else
bothered to look at the 1906 paper.
But John did look at it. Then everything came
clear. This century-old paper gave the only
measurements of viscous slowing anyone had ever
made. It was beautiful work. Maybe the contraction
measurements were flawed. But it was now
crystal-clear that viscosity had almost no effect
After 1940, textbook writers misread the 1940 paper
without looking back at the 1906 paper. Now every
textbook says that two percent of the speed is
lost. Every textbook is wrong.
That's what happens when we forget primary sources
-- when we bury the past. It's a story we can tell
a hundred different ways. This kind of thing
happens all the time.
We face a hard problem when we try to do something
original and new. We have to shed the past to
create the future. Yet the past contains such
riches of knowledge. The most pernicious myth about
knowledge is that it deteriorates with age. It does
not, of course. And, when we lose the ability to
remember, we defile what we do today. We rob our
own best effort of its future.
That's an essential tension we face as we go after
new knowledge. How do we use the past without
letting it turn us into a pillar of salt? How do we
think forward and look back at the same time?
That's not an easy question. It's not easy at all.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
Lienhard, J.H., V, and Lienhard, J.H. (IV), Velocity
Coefficients for Free Jets from Sharp-Edged Orifices.
J. Fluids Engr., Vol. 106, No. 1, 1984,
Medaugh, F.W., and Johnson, G.D., Investigation of
the Discharge Coefficients of Small Circular
Orifices. Civil Engr., Vol. 7, No. 7,
1940, pp. 422-424.
Judd, H., and King, R.S., Some Experiments on the
Frictionless Orifice. Engr. News, Vol.
56, No. 13, 1906, pp. 326-330.
The engineer who made the remark about primary
sources is Neal Amundson, Professor of Chemical
Engineering at the University of Houston. Amundson
is the pioneer of modern mathematical technique in
For more on this theme, see episode No. 597. We also discuss this idea
further in a note under review: Myers, J.E., and
Lienhard, J.H., The Fate of Knowledge.
From the Lienhard-Lienhard paper
The configuration of a jet leaving a sharp-edged
Velocity loss in an orifice
The Engines of Our Ingenuity is
Copyright © 1988-2004 by John H.