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.

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.