Long ago our ancestors discovered how to use simple tools--such as hammers, shovels, crowbars and rollers. These things seem very ordinary to us, but they were really wonderful discoveries. The clever men who invented them were providing ideas, one by one, which scientists and engineers used much later. Our great machines are combinations of many, many things that men discovered from using simple tools. POWERFUL PUSHERS The giant shovel digs; the overhead crane lifts; the pile driver pounds. All machines multiply the power that’s in the muscles of men--or of animals. The pushingest animal is an elephant. In some places in the world, elephants are trained to clear land by putting their foreheads against a tree and heaving until the tree topples over. A tree-dozer can out-push an elephant. The one in the picture has a special forehead built in front. With a slow, steady shove, it clears the way for roads or opens up fields for farms. [Illustration: tree-dozer] Farmers used to dig their fields by hand. Then they hitched horses to plows. Now a tractor does the work, but we still measure its strength in horsepower. [Illustration: elephant] MACHINES FOR FARMERS Dan is a farmer. He knows how to use almost any kind of farm machine, and he has lots of them. The most important is his tractor, for it is busy all year round. Sometimes it pushes. Sometimes it pulls. Or it may stand still and lend its power to other machinery. When the frost is out of the soil in the spring, Dan backs his tractor into the tool shed and bolts on a plow. This one is a two-gang plow--it can make two furrows in the earth at the same time. Dan touches a lever. The blades of the plow lift up so they can’t dig into the farmyard and the road, and Dan chugs off to the field. Another touch on the lever sends the blades down. In a few minutes, Dan has made the first furrows across the field. Now he has to turn. He lifts the plow and steps on the left brake pedal. While the big left wheel stands still, the right one keeps going and turns the tractor, ready to start the next furrows. When Dan wants to stop, he steps on both the left and right brake pedals at once. After plowing comes harrowing. The tractor pulls a different implement for this job--a whole row of saucer-shaped metal discs that chew up the soil and spread it out evenly. Now Dan is ready to plant corn. [Illustration: harrowing disc] [Illustration: corn planter rear]