Mountain rivers flow at superhigh speeds because they are steep. The rivers strip away bits of rock and carry them downhill.
After thousands of years, the river strips away enough rock to form a canyon—a wide, deep crack in solid rock. If water can carve a mighty canyon, imagine what water power can do for you. People put water to work long ago—to move heavy objects, to travel down rivers and across oceans, and even to keep time. Here are just a few of the water-powered machines and systems that humans have built.

◀ Water clocks have been around for thousands of years. This fifth-century B.C. clepsydra (water clock) may have been based on Egyptian clocks of the fourteenth century B.C. If so, it was probably filled with water every evening, to be used as a way of telling the time during the night. The water flowed gradually through a hole near the base. The passage of time was measured by how far the water had dropped below the “hour” marks visible inside.

▲ Hydrofoils are boats with underwater wings, called foils, which act like airplane wings. The water that passes above the foils is moving faster than the water traveling below them. This faster water reduces the pressure above the foils. So, the slower water presses up, creating a force called lift, which literally lifts the hydrofoil above the water.

▲ Water mills work like windmills, except they are powered by moving water—not wind. The ancient Romans used water mills 2,000 years ago. Water flowed downhill, forcing a series of paddle wheels to turn, and those wheels rotated gears that powered machines. The machines ground corn, among other jobs.

◀ Pyramid builders of ancient Egypt poured buckets of water onto a mud-and-rubble ramp to make it slippery. Behind them came workers dragging two-ton blocks of stone strapped to wooden platforms. The slippery ramp let the platforms glide more easily to the top.
Fuel cells combine hydrogen and oxygen to create water and electricity. In space, they provide astronauts with drinking water and energy. On Earth, some cars and buses use fuel cells for power. The driver “fills up” the vehicle’s tank with hydrogen gas, and the fuel cells take in oxygen from the air. The water is drained off, and the electricity is used to power the vehicle. ▶


Steam Power
Here’s how steam-powered ships of the nineteenth century used water to travel on water. ▼

The motion of the piston causes other parts of the furnace to move. These different components help to turn a giant paddle wheel. The paddle wheel pushes water backward, which pushes the steamship forward.
The pressure from the water vapor forces the piston to move out. As the steam cools and condenses to become water again, the piston drops back in.
Heat from the furnaces boils water, and the water is converted to steam. When the steam is heated up even more, the water vapor begins to expand. It exerts pressure against the movable part of the furnace, called the piston.
Below decks, the ships carry huge stores of coal in their holds. Sailors constantly shovel the coal into hot furnaces.