Today, let's ask what meters really measure. 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.

When the Greek philosopher, Protagoras said, "Man is the measure of all things," he was closer to literal truth than we realize. The gauges and meters we use to measure things usually begin by copying our own senses. You can see that by looking at our weights and measures, which largely reflect what we see and feel.

For example, a pound or even a kilogram is roughly the mass of any fairly dense material -- like a rock or a piece of metal -- that we can hold comfortably in our hand. The inch, foot, yard, and meter all correspond roughly with various body parts.

The mile and kilometer also have a meaning that's made clear in parts of rural America, where people talk about the distance of a see. Ask someone in eastern Kentucky how far it is into town, and he might say, "Oh, 'bout two sees." He means you should look down the road as far as you can see. Where your vision runs out, you spot -- say -- an oak tree. You walk to it, and look again. There in the distance, is the town -- just two "sees" away. And how far is a see? Of course it varies; but even in flat terrain, our ability to make things out usually ends after about a mile or a kilometer.

We divide our thermometers into degrees Fahrenheit or Celsius, and these are roughly the smallest increments of temperature we can feel. We usually know if we have a one-degree fever. We can sense about one volt with our tongue; our ears are sensitive to about one pound per square inch of pressure change; and so on.

The kilowatt and horsepower are roughly the power that most of us can produce in a short sprint -- like running upstairs. By the way, when James Watt specified the unit of a horsepower, he made it less than the work of a real horse so his engines would seem more powerful. Not only is the kilowatt or horsepower close to the maximum power you or I can produce; it's also the most power we can tangle with without being hurt -- the rate the sun pours energy on us if we tan ourselves on the beach, or the rate we consume energy when we take a hot shower.

Since we're the basis for most of our measuring devices, our science reflects the world in human terms. But that's not really so bad. Scientists realize that science isn't ultimate truth. It's something we construct to make our experiences predictable. Our science-based engineering obviously serves us by mirroring human needs and nature. And so does science itself.

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

(Theme music)

This episode has been substantially revised as Episode 1363.