As far back as the Middle Ages, engineers were consumed with the challenge of developing non-human forms of power that would ease the burdens of workers while at the same time making available more comforts and conveniences for a greater number of people. The major sources of such power were animals, wind, water, and other natural forces, harnessed to drive an increasing variety of devices and inventions, many of them remarkably ingenious and efficient. Modern machinery derives in part from the primitive quern, which was used for grinding grain, and applied the force of two millstones that rotated upon each other. Gears and cogwheels were utilized to increase the power of machines operated by non-human forces. And more sophisticated variations of the ancient windmill demonstrated engineering innovations like tilted sails to catch the wind more effectively, and multiple axles and gears that would utilize more power while driving pumps and other machinery.