Steep mountain rivers flow at superhigh speeds. The water strips away bits of rock and carries them downhill.
After thousands of years, the river strips away enough rock to form a canyon. That’s a wide, deep crack in solid rock. If water can do that, imagine what else it can do. People put water to work long ago. They used it to move heavy objects. They used it to travel down rivers and across oceans. They even used it to keep time! Here are just a few of the water-powered machines and systems humans have built.

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

▲ Hydrofoils are boats with underwater wings, or foils. The foils act like airplane wings. The water passing above the foils moves faster than water traveling below them. The faster water lowers the pressure above the foils. So, the slower water presses up. It creates a force called lift. And this force literally lifts the hydrofoil above the water!

▲ Water mills work like windmills, but they are powered by moving water, not wind. The ancient Romans used them 2,000 years ago. Water flowed downhill and made a series of paddle wheels turn. The wheels turned gears that drove machines. The machines did many jobs, including grinding corn.

◀ The pyramid builders of ancient Egypt used water in their work. They poured buckets of it onto a mud-and-rubble ramp. The water made the ramp slippery. Behind them, workers dragged two-ton blocks of stone strapped to wooden platforms. The slippery ramp let the platforms slide more easily to the top.
Fuel cells combine hydrogen and oxygen to make water and electricity. In space, they give astronauts 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. The fuel cells take oxygen from the air. The water drains off, and the electricity powers the vehicle. ▶


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

The motion of the piston makes other parts of the furnace move. These parts help to turn a giant paddle wheel. The wheel pushes water backward, which pushes the ship forward.
The pressure pushes the piston out. As the steam cools and condenses into water, the piston drops back.
Heat from the furnaces boils water. The water is converted to steam. The steam gets heated up even more, and the water vapor expands. It puts pressure against the moving part of the furnace. That’s called the piston.
Below decks, the ships carry big piles of coal. Sailors shovel the coal into hot furnaces.