Boats need a power source to move them forward in the water. In small vessels this power can be provided by people, who use oars to paddle along. Muscle power cannot move boats very fast or very far, though. The wind can be used, too, to move boats equipped with sails. But for a large boat that needs to go a long distance, the most reliable source of power is a motor-driven engine. Depending on the size of the boat, a gasoline engine, diesel engine, or steam engine does the job. Nuclear power is even used to run some boat engines, like those found in submarines. Motors rotate boat propellers, which have large twisting blades that radiate around a central hub.
These blades push water backward, and the boat moves forward as the disturbed water pushes back. Rotating propellers also create lower water pressure in the space in front of them, which sucks them forward, along with the vessel to which they are attached. (Using these same principles of movement, propellers can also power aircraft.) A boat is steered by a rudder, Which is a flat, upright, movable piece of wood or metal that is attached to its stern, or rear. When turned, the rudder changes the direction of the water around it, which pushes back, forcing the stern, and gradually the rest of the boat, to change direction, too.
Living species of sequoias are some of the biggest and widest trees in the world. The tallest trees are the coastal redwoods of California, while the widest ones are the giant sequoias, which have much larger trunks and branches. These trees grow in the Sierra Nevada mountains of Central California at elevations of about 6,500 feet (1,981 meters). The trees have survived for 2,000 or 3,000 years. Some of the largest sequoia trees measure 35 feet (10.6 meters) in diameter and up to 300 feet (91 meters) in height.
Their bark can be 4 feet (1.2 meters) thick! Many are found in California’s Sequoia National Park. The General Sherman, for example, is one of the tallest giant sequoias in the world, with a height of about 275 feet (85 meters).
Tropical coral reefs are ridgelike or moundlike structures composed of corals and other aquatic organisms. They border the shorelines of more than 100 countries. Although coral reefs comprise less than 0.5 percent of the ocean floor, it is estimated that more than 90 percent of marine species are directly or indirectly dependent on them—they are home to approximately 4,000 species of fish alone.
Reefs protect human populations along coastlines from wave and storm damage by serving as buffers between oceans and near-shore communities. The Great Barrier Reef, located in northeast Australia’s Coral Sea, measures 1,616 miles (2,000 kilometers) in length. It the largest living structure on Earth, and can be seen from the Moon.
Television works through a series of complicated processes. It starts with a television camera, which takes pictures of scenes. Photo cells inside the camera change the pictures to electrical signals. At the same time, a microphone records sounds that are occurring during the scenes. A vibrating magnet in the microphone changes these sounds into electrical signals, too. Some television shows, like news reports, are recorded live, which means that they are broadcast to homes as they occur. But most of the television programs that we watch are recorded, which means that they are put on videotape and sent out later. The electrical signals of sound and pictures are stored as magnetic signals on videotape, which are converted back to electrical signals when played. Before a program is broadcast, its electrical picture and sound signals are run through a device called a television transmitter. With the help of strong magnets, the transformer turns the electrical signals into invisible bands of energy called radio waves (similar to visible light waves), which can travel great distances through the air. They can travel directly to outdoor television antennae, which catch the waves and send them to television sets that change them into pictures and sounds again.
Cable companies send electrical picture and sound signals through cables directly to homes. When broadcasting to distant places, communication satellites that orbit Earth are used to bounce or return the waves back to Earth, extending their travel distance. Satellites are necessary because radio waves move in straight lines and cannot bend around the world. When an antenna or satellite dish receives radio waves, it changes them back into electrical signals. A speaker in a television set changes some of the signals back into sound. The pictures are reproduced by special guns at the back of a television set that shoot electron beams at the screen, causing it to glow with tiny dots of different colors. Viewed together, the dots look like a regular picture. The individual pictures that make up a scene are broadcast and received, one after another, at a pace so quick that it looks like continuous action is occurring on the screen. The entire process happens very fast because television stations and broadcast towers are all around and because radio waves travel very quickly, at the speed of light. Radio programs broadcast talk and music across the airwaves using the same technology.
Scientists believe that the solar system in about 4.6 billion years old. Earth and the rest of the solar system formed from a giant cloud of gas and dust. Gravity and rotational forces caused the cloud to flatten into a disc and much of the cloud’s mass to drift into the center. This material became the Sun. The leftover parts of the cloud formed small bodies called planetesimals.
These planetesimals collided with each other, gradually forming larger and larger bodies, some of which became the planets. This process took approximately 25 million years, according to scientific estimates.
Yes. The air inside your home might be filled with toxins from tobacco smoke, cleaning products, ceiling tiles, and upholstery. Scientists have discovered that many types of houseplants absorb airborne pollutants as part of their normal “breathing” process—they take carbon dioxide in through their leaves, and let oxygen out. The plant transports these toxins to their roots, where microbes feed on and detoxify them.
Although scientists disagree about how many—and what types of—houseplants it takes to clean the air, they suggest using a mix of plants. Bill Wolverton, a former NASA scientist and environmental engineer, studies the effects that plants have on air quality and has rated the areca palm, lady palm, bamboo palm, rubber plant, and dracaena as highly effective at clearing pollutants from the air.
In 1943 and 1944, the British government developed two Colossus computers. These huge machines were electronic computing devices used by British code breakers to read encrypted German messages during World War II. Dubbed “Colossus Mark 1” and “Colossus Mark 2” these devises were the world’s first programmable, digital, electronic, computing machines.
Based on concepts of the British mathematician Alan M. Turing, the mathematician Max Newman and engineer Tommy Flowers designed and built the machines, which used vacuum tubes (thermionic valves) to perform the calculations. The Colossus hardware and blueprints were destroyed as part of an effort to keep the project secret. However, based on notes in engineers’ logs and other information, in 2007 a functional replica of a Colossus computer was completed. The computer is on display at the Bletchley Park Museum in Milton Keynes, Buckinghamshire, England.
Most plants have leaves, even if they do not look like leaves. For example, blades of grass are really leaves. Mushrooms and other fungi do not have leaves, and seaweeds and lichens do not have leaves. Seaweed, a type of algae, also does not have flowers or roots. As an underwater plant, it usually clings to stones, shells, and rocks with its holdfast, a part of the plant that looks like roots. Unlike other plants that feed through their roots, seaweed takes its nutrients from the water in which it grows.
Scientists do not know exactly why people need sleep, but studies show that sleep is necessary for survival. Sleep appears to be necessary for the nervous system to work properly. While too little sleep one night may leave us feeling drowsy and unable to concentrate the next day, a long period of too little sleep leads to poor memory and physical performance.
Hallucinations (seeing things that aren’t really there), vision problems, and mood swings may develop if sleep deprivation continues.
An avalanche is a huge mass of ice and snow that breaks away from the side of a mountain and slides downward at great speed. Most avalanches result from weather conditions, such as heavy winds and earth tremors, that cause snow on a mountain slope to become unstable.
A large avalanche in North America might release 300,000 cubic yards of snow—the equivalent of 20 football fields filled 10 feet (3.3 meters) deep with snow. Wintertime, particularly from December to April, is when most avalanches occur.