Euclid’s Elements is a series of 13 geometry and mathematics books written by the Greek mathematician Euclid in Alexandria about 300 B.C.E. It is a collection of definitions, postulates (axioms), theorems, and mathematical proofs of the propositions. The 13 books cover Euclidean geometry and the ancient Greek version of elementary number theory. Along with the Greek mathematician Autolycus’s On the Moving Sphere, the Elements is one of the oldest Greek mathematical treatises to have survived, and is the world’s oldest continuously used math textbook.
Historians do not know a lot about Euclid’s life, but his work has proven important to the development of logic and modern science. Most of the theorems in the Elements were not discovered by Euclid himself, but were the work of earlier Greek mathematicians such as Pythagoras, Hippocrates of Chios, Theaetetus of Athens, and Eudoxus of Cnidos. However, Euclid is credited with arranging these theorems in a logical manner.
Dust mites are microscopic organisms that live in dust. These unwelcome visitors invade your nose and can irritate your mucous membranes, triggering nerve cells that signal the lungs to fill with air. When the air passages close and pressure builds up, your nose tingles and twitches, and you sneeze—forcing mucus (the slimy, moisturizing substance), dust, pollen, and mites out of your nose at speeds of up to 525 feet (160 meters) per second!
Sneezing is one of the body’s reflexes, an automatic way it rids itself of harmful substances like bacteria and germs. It also keeps the tubes that carry the air from your nose to your lungs healthy.
As a matter of fact, yes! Researchers believe that regular contact with pets can reduce levels of stress and reduce blood pressure (the force of blood pushing against the walls of the arteries as the heart pumps out blood).
Pets offer stability, comfort, security, affection, and intimacy. Owning a dog also provides a great opportunity to get exercise and fresh air, since it will need to go for a walk every day.
A fruit is the part of the plant that nourishes and protects new seeds as they grow. The plant’s ovaries develop into fruit once the eggs inside have been fertilized by pollen. Some plants produce juicy fruit, such as peaches, pears, apples, lemons, and oranges.
Others produce dry fruit, such as nuts and pea pods. If an animal doesn’t eat the fruit, or a human doesn’t pick it off, it falls to the ground and decays and fertilizes the soil where a new seed will grow.
Birds migrate—or move regularly from one place to another—for several reasons, including warmth and the availability of food and water. Many species of birds mate and nest in specific areas of the world. Most of these areas are only comfortable during the warmer months of the year, so when the cold weather arrives birds migrate to warmer climates. These trips can be as long as thousands of miles. For example, the American golden plover breeds north of Canada and Alaska during the Northern Hemisphere’s spring and summer.
In the Northern Hemisphere’s fall, the plovers travel to southeastern South America to spend the “winter”—which is the summer season in the Southern Hemisphere—allowing the birds to find plenty of food. When spring arrives again in the Northern Hemisphere, the trip is reversed, and the plovers migrate back to the northern nesting grounds to breed.
Moss is a type of plant that does not have traditional roots, stems, or leaves. Because they have no true roots, mosses use delicate growths called rhizoids to anchor them to soil, rocks, or tree bark. Moss grows along the ground where it is moist, absorbing water and nutrients from the air.
Like their cousins the ferns and liverworts (leafy mosses), mosses reproduce from spores, not seeds, and need to be moist in order to reproduce. They grow in soft cushions or small clumps, and can spread out like a blanket along the ground.
Kissing bridges are covered bridges with roofs and wooden sides. They are called kissing bridges because people inside the bridge cannot be seen from the outside, making them good places to kiss discreetly. They were first built in the nineteenth century by engineers who designed coverings to protect the structures from the effects of the weather.
More than 10,000 covered bridges were built across the United States between 1805 and the early twentieth century. As of January 1980, only 893 of these covered bridges remained—231 in Pennsylvania alone, where the first one was erected.
The ribs are thin, flat, curved bones in your upper body that form a protective “cage” around the heart and lungs. The ribs are comprised of 24 bones arranged in 12 pairs that form a kind of cage that encloses the upper body and gives the chest its familiar shape. The ribs serve several important functions.
They protect the heart and lungs from injuries and shocks that might damage them. Ribs also protect parts of the stomach, spleen, and kidneys. The ribs help you to breathe. As you inhale, the muscles in between the ribs lift the rib cage up, allowing the lungs to expand. When you exhale, the rib cage moves down again, squeezing the air out of your lungs.
In 1823, the English mathematician originated the concept of a programmable computer. At this time, he persuaded the British government to finance what he called an “analytical engine.” This would have been a machine that could undertake any kind of calculation. It would have been driven by steam, but the most important innovation was that the entire program of operations was stored on a punched tape (a long strip of paper in which holes are punched to store data).
Babbage’s machine was not completed in his lifetime because the technology available to him was not sufficient to support his design. However, in 1991 a team lead by Doron Swade at London’s Science Museum built the analytical engine (sometimes called a “difference engine”) based on Babbage’s work. Measuring 10 feet (3 meters) wide by 6.5 feet (2 meters) tall, it weighed three tons and could calculate equations down to 31 digits. The feat proved that Babbage was way ahead of his time, even though the device was impractical because one had to a turn a crank hundreds of times in order to generate a single calculation. Modern computers use electrons, which travel at the speed of light.
While some birds eat mostly insects, others, like penguins, eat seafood. Beach birds, like seagulls, eat shellfish as well, but they are also scavengers that will eat discarded people food. Some birds, such as ducks and geese, float on the water, dipping or diving to nibble on plants from oceans, lakes, and rivers. Others, such as raptors, swoop out of the sky to capture and eat small mammals, such as mice or rabbits. Some birds also prey on each other, such as large predatory birds like eagles and hawks.
Many birds, like crows, jays, and magpies, eat the eggs and young of others. Individual bird species eat the foods from their local environment, but they have also developed physical characteristics that help them harvest food. Specific birds have adapted to feasting on plants as well, including algae, lichen, grass, herbs, flower nectar, leaves and buds of trees, ferns, acorns, nuts, corn, rice, and seeds of all kinds.