Today, we find it's harder than we thought to imitate nature. 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.

Our machines mirror living beings. We made the first tractors with the horse in mind. Computers copy the lower functions of our own brain. We were looking at birds as we struggled to invent the airplane. That mimicry gives our machines a certain intimacy with us. But it also limits what they can do.

When designers first made robotic arms, they had to unlearn what they knew about their own arms. The movement of machine joints needn't be limited the way our elbows and knees are. Remember how horrified you were when a little girl's head turned all the way around in The Exorcist? That'd be easy for a robot.

Look at the airplane: Down through history, we tried to fly by copying birds. That very fact kept us on the ground for a long time. We finally had to see that birds were far too complicated to copy -- at least in this stage of our technology.

Most early airplane designs were ornithopters, driven by flapping wings. To make a wing propel us at the same time it lifts us is far harder than it looks. Birds do it with very complex motions. When we finally did manage to combine lift and drive in a wing, it was in the far simpler helicopter. We've yet to make a workable ornithopter. And we didn't even learn to make helicopters until a generation after the Wright Brothers flew.

Birds don't have vertical tails, so at first we didn't put vertical tails on airplanes to stabilize their motion. But birds constantly and quickly adjust their direction in flight. No early airplane could be controlled with that delicacy. Birds have high wings, and they land with their tails down. So it was with the first generation of airplanes. To learn flight, we've had to shed our knowledge of birds, one feather at a time.

We've also failed to make submarines that swim like fish, vehicles that move like animals, or computers that think like humans. The reason is that nature does these things in ways that are -- so far -- too complex to copy.

Did you know that engineering designers are developing a six-legged walking vehicle for the army? It's still primitive. We haven't yet mastered four- or two-legged vehicles. In that sense we've evolved no further than the insects. The human brain remains a far distant target for computer engineers.

Perhaps we shall, one day, come full circle. We might yet learn to fly with the grace and delicacy of a bird, or swim like a fish. It's much harder than it looks to copy nature, but perhaps, someday, we shall.

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

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Harris, J.S., The Airplane is Not a Bird. American Heritage of Invention and Technology, Fall 1989, pp. 18-22.