Today, "Man may work from sun to sun, but woman's work is never done!" 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.

Historian Ruth Schwartz Cowan offers a remarkable idea. It is that homemakers today work about as long as homemakers did in the 19th century -- still about 50 to 60 hours a week. The load only varies slightly with the number of children being raised.

Our homes are served by scores of electric motors -- thousands of computer chips. Yet homemakers work just as long. The big revolution hit between 1920 and 1960 with gas and electric heaters, washers, driers, refrigerators, vacuum cleaners, and mixers.

When I was a kid in the `30s, I had to grind the coffee in a hand mill. Now there was a terrible job. After ten minutes of that, my arm ached. Every function took more work. Shoveling coal and taking clinkers out with tongs, instead of just turning up the thermostat -- sweeping rugs, putting up storm windows. How many of you are old enough to remember the old wire rug beater?

So why hasn't the workload fallen? Cowan looks at details of, say, making a bed. Once we moved the top sheet to the bottom and put only one sheet in the wash. Once we changed underwear weekly. Now most of us change it daily. It doesn't take more time to sweep a rug than it does to vacuum it. But the vacuum leaves the rug far cleaner.

Automobiles gave us mobility. So doctors and grocers soon made patrons come to them. Homemakers became chauffeurs. Cowan tells how, as she wrote her article on housework, she had to stop to take a child to a hockey game, bring her home, pick up groceries, get her husband at the train station and visit the doctor's office.

So what about women entering the work force? Well, Cowan is a distinguished historian juggling children and home. She reminds us that, when both spouses work, they're still left with some 35 hours of housework. Couples are learning to divide that labor. They also shift some of the load to people who make their living in food services, condo managing, and child-care. But work remains.

One of Murphy's laws says work expands to fill the time. But this is different. For our lives are now cleaner and healthier. And, when we lay aside maudlin nostalgia, we see our lives are probably richer as well.

"What's the last appliance you'd give up?" I ask my wife. "The refrigerator," she says without missing a beat. "Think of daily trips to the store, the rotting food." Cowan, who's still raising children, points to the quality of medical care -- the burden of fear it removes from a young mother. A woman, she says,

may be exhausted at the end of her double day, but the modern "working" housewife can at least fall into bed knowing ... her efforts have made it possible to sustain her family at a level of health and comfort [once] reserved for [the] very rich.

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

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Cowan, R.S., Less Work for Mother? American Inventions:A Chronicle of Achievements that Changed the World. New York: Barnes & Noble, 1995, pp. 159-165.