All energy is either the direct or indirect capture of solar energy. Chapter 1 focused on the organic economy of human and animal muscle and wood. In the case of humans and animal muscle, some movement is possible but at great expense and with diminishing returns. In Chapter 2, the energy derived from windmills and watermills operates on the principle of motion. Motion means kinetic energy, whether its air or water in motion. The particles of air are gaseous while those in water are liquid. Capturing the movement of one in a windmill or the other in a watermill means capturing their energy and harnessing it to do work. Wind and water are forms of solar energy. For wind, it’s the heating of the atmosphere by the sun. For water, it’s the planet’s rotation and the irregularities of Earth’s surface. Energy from wind and water replaced humans and animals from much of the drudgery and monotony of repetitive activity. Their use declined rapidly during the modern era.