Steve Groat, age 10, of Wichita, Kansas,for his question:

How does a windmill work?

Wind is moving air and moving air has pushing power. You can watch the gentle force of a breeze flap a fluttering flag. You can feel the mighty driving force of a strong gale. Windmills were invented to tame the force of the wind and train it to perform useful chores.

One of the toughest chores of early days was the grinding of wheat into powdery flour. The job was done by placing the hard grains of wheat between two flat,round millstones and turning the top stone around by muscle power, It was, we think, the Saracens of the Middle East who first invented a labor saving device to perform this milling work. Instead of muscle power, they used the energy of the wind. Early windmills were made ages ago to grind corn and also to pump water. The most up to date windmills use the energy of the huffing puffing wind to turn electric generators.

Through the centuries, many kinds of windmills have been designed. But all of them work more or less on the same basic plan. A circle of rectangular sails is hoisted high in the air. The sails are set to catch the wind so that it blows them around and around. They are fixed to a central horizontal shaft and as the sails spin around, the shaft rotates. The rotating shaft has torque, or turning energy, and mechanics can do a great deal with this torsion energy.

With gears and interlocking toothed wheels, they can make the torsion energy change directions and move up and down. The shaft that is turned by the spinning sails of a wind¬mill is fitted with a toothed wheel. Its teeth interlock into the teeth of a larger wheel. As the wind blows, the sails turn the shaft that turns the first wheel that turns the larger wheel. Other gears may be interlocked to turn a huge shaft inside the mill house.

The wind driven clockwork of the mill may be harnessed to turn a heavy stone to grind grain. Or it may be geared to make a pump haul up water from the ground. Some of the water is led off in pipes. Other clockwork in a water windmill may pipe some of the .water into a storage tank. Electric power is generated by turning copper coils through a magnetic field. A windmill can be designed to turn a generator and send current through a copper circuit. Outlying farms often use windmills to pump water and to generate electric power for home use. In populated areas these jobs are done more easily with steam power or with the energy of falling water. And nowadays, almost nobody uses a windmill to grind grain into flour.

The wind, of course, is famous for changing directions and in older days this was quite a problem. The miller's helper had to walk the sails around into position to catch the breezes. Sometimes they were fixed to turn around with the dome on top of :the mill tower. Sometimes the whole mill house was built to turn around with the sails. A modern windmill is fitted with a big, flat rudder that works like a movable weather vane, turning the circle of sails into the windy direction.