Today, a strange parable of appearance and reality. 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.

Roller skates are odd gadgets, if you think about it. Each skate is a small four-wheeled platform. A single roller skate isn't enough to provide a stable base. After all, you can't really stand on a skate. When you move forward, you fall from an unstable perch on one leg to catch yourself on the other.

In that sense, roller skates are like bicycles. The modern safety bicycle was finally perfected in 1885 after 50 years of experimentation. You're never stable on a bike. You're constantly just starting to fall and correcting your motion.

So the four-wheeled roller-skate platform is only an illusion. Eliminate the illusion -- put the wheels in a single row -- and you gain fluidity and control. All you lose is the stability you never had in the first place. Of course, when you do that, you get the in-line roller blade -- something you see everywhere today.

Now the surprise: The first roller-skate patent was issued in 1819. The patent drawing shows three wheels in a line. The roller blade came before the four-wheeled platform skate!

Yet it was 1981 before Scott Olson founded Rollerblade, Inc., in Minnetonka, Minnesota. Olson wanted to keep up his ice hockey skills during the summer. Since that beginning, roller blades have gone through a dizzying series of experimental designs. They're now made with from two to five wheels and complex arrays of bearings and braking systems.

Leo O'Connor writes about this design profusion in Mechanical Engineering magazine. Roller-bladers cruise at around ten miles an hour -- a lot faster than the old roller skates. An expert will reach 50 mph on a downhill run. Once released from the idea of a platform, we've become drunk on a new concept of motion. Yet there's an odd historical continuity in the story of the roller blade.

Eighteen years after the modern bicycle took its form, Orville and Wilbur Wright took the concept of instability to Kitty Hawk, North Carolina. There, in the coastal dunes, they finally removed the flawed concept of stability from the airplane. They created an unstable airplane whose motion had to be controlled in flight. Only then did they succeed where everyone else had failed.

And I'm left puzzled. Why did it take a century? It's a reminder of the way we hamstring invention by assuming illusory constraints. Just think: we've used no more than the illusion of stability to protect generations of children -- as they ride about with skinned knees, on their four-wheeled platform skates.

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

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O'Connor, L., Technological Developments Keep Skaters In-Line. Mechanical Engineering, Vol. 117, No. 8, August 1995, pp. 82-84.