Today, thoughts about cities, farms, and consumption. 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.

The other night I sat in a meeting of environmental historians. They spoke of cities and farms. They agreed that cities increase consumption. Finally, I rose to object. Cities don't increase consumption. They reduce it.

It seemed so obvious to me -- yet they did not agree. So let me try it on you. How do cities relate to consumption?

Clearly, we consume more goods and energy per square mile in a city than on a farm. But consumption per square mile isn't important. What kills us is consumption per person. Both our population and the goods each of us uses are rising. Put the two together, and we stress our Island Earth very seriously.

So how do the people of New York and Tokyo use their resources? They ride mass transit. They live and work in centrally heated buildings. Centralized dining wastes less food. City dwellers carry out every function of daily life more efficiently. The huge power plants that supply cities are far cleaner per kilowatt hour than wood-burning stoves or kerosene lamps.

That efficiency has a sad side effect. The poor go to cities. The efficiency of cities makes survival possible for those least able to consume.

Yet on another level my historian friends are right. Cities are the great bazaars of our society. We display our wares in cities. They are where our wants are stimulated. A farmer might own the same television I do. He might drive a larger car. Cities drive us both to want those things.

You see, cities and farms are not separate things. They're different faces of the same large system. Neither could survive without the other. Neither would even exist without the other. The city-farm system expresses what we are. Our machines and our poetry -- our music and our thoughts -- are shot through with imagery of cities and farms.

Cities and farms together lead us to consume more. On the other hand, the very size of cities constantly pushes us to reduce waste.

The real cause of consumption is not the city itself. It's our childlike craving for novelty and freedom from constraint. Those forces have always driven technology. I suppose they always will. An adult takes time to savor what he has. But a child demands more. And that's why cities sometimes look more like Dante's vision of Hell than the latter-day Edens they can become.

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

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