Science & Philosophy: Two-envelope paradox to Venus
Tyrannosaurus rex
Tyrannosaurus rex or T. rex, is easily the most famous of the Tryannosaurids (tyrant reptile). Despite its popularity, T. rex appears to have had a limited range in North America and Asia, and existed for a relatively short period of time. T. rex appeared during the late Cretaceous Period, about 85 million to 65 million years ago. This was toward the end of the Mesozoic Era or the Age of Reptiles,…
Tyrant Flycatchers - North American species of tyrant flycatchers
The tyrant flycatchers are a large family of perching birds, containing 367 species, and making up the family Tyrannidae in the order Passeriformes. Tyrant flycatchers only breed in the Americas, from the northern boreal forest of Canada, through the rest of North America, Central America, and to South America as far south as Patagonia. Rarely, individual tyrant flycatchers may occur in coastal Eu…
Ultracentrifuge
An ultracentrifuge is a mechanical device that separates substances of different densities by spinning them very fast. It greatly reduces the time it would take to separate substances that would eventually separate if left alone. The first successful centrifuge was invented in 1883 by Swedish engineer Carl de Laval. It was used to separate cream from milk. Forty years later, another Swede, chemist…
Ultrasonics - How Ultrasonic Waves Are Generated, Ultrasonic Dispersion, Ultrasonic Cleaning, Welding, Nondestructive Testing, Scientific Research - Applications, Coagulation, Humidification, Milk homogenization and pasteurization, Drilling, Soldering, El
Ultrasonics or ultrasound, derived from the Latin words "ultra," meaning beyond, and "sonic," meaning sound, is a term used to describe sound waves that vibrate more rapidly than the human ear can detect. Sound waves travel as concentric hollow spheres. The surfaces of the spheres are compressed air molecules, and the spaces between the spheres are expansions of the air…
Underwater Exploration - History, Oceanography, Instrumentation, Diving Tools And Techniques, Deep-sea Submersible Vessels, Key Findings In Underwater Exploration - Deep-sea pioneers
Underwater exploration is the relatively recent process of investigating the depths of the sea to understand its physical and chemical characteristics and to learn about the life forms that inhabit this realm. Underwater exploration near the surface and near the shore is an ancient form of earning a livelihood and enjoying the pleasures of the water; but deep-sea exploration is a recent phenomenon…
Ungulates
Ungulates are large grazing animals whose toenails have become enlarged into hooves. There are two orders of ungulates: Perissodactyla and Artiodactyla. Animals in the order Artiodactyla have an even number of toes (usually two) that form a cloven hoof. This order is relatively diverse, containing 82 genera and several hundred species. There are nine families in this order, the most familiar of wh…
Uniformitarianism
Uniformitarianism is commonly oversimplified where stated in geological textbooks as "the present is a guide to interpreting the past" (or words to that effect). This explanation, however, is not correct about the true meaning of uniformitarianism. In order to understand uniformitarianism, one must examine its roots in the Enlightenment era (c. 1750–1850) and how the term has …
Units and Standards - History, The Metric System, Le Système International D'unités (the Si System), Derived Units
A unit of measurement is some specific quantity that has been chosen as the standard against which other measurements of the same kind are made. For example, the meter is the unit of measurement for length in the metric system. When an object is said to be 4 m long, that means that the object is four times as long as the unit standard (1 m). The term "standard" refers to the physical…
Universalism - Bibliography
Since the late nineteenth century, the debate around issues concerning universalism and universalizability has intensified. Against the claims to universal knowledge made on behalf of Christianity, the West, rationality, and mankind, feminist, critical race, and postcolonial scholars and activists have shown that the issues are more complicated. Notwithstanding the validity of their criticism, uni…
University - Postcolonial - A Contrasted Picture, Indian Higher Education System: The Crippled Giant, A Peripheral World Of Learning
The late twentieth century saw a renewed interest in the postcolonial development of higher education systems within broader literature on globalization and education policies. Particularly, efforts by international institutions, such as the World Bank, to prevent the global and local effects of the so-called knowledge divide, led to a number of policy documents and initiatives aimed at leveling t…
Untouchability - The Origin Of Untouchability, The Voices Of Untouchables, The Beginnings Of "affirmative Action", The Contemporary Period
Parallel to the varnas and outside scripture were jatis, meaning "by birth" and also translated as castes. A jati is an endogamous group, sharing many customs and often an occupation, usually based in one language area. There were hundreds of jatis within each varna, and while untouchables were avarna, without varna, they were members of specific jatis. …
Untouchability - Menstrual Taboos - Menstrual Taboos In Tribal And Band Societies, Menstruation And Civilization, Medical Variations And Modern Interpretations
Menstruation is a physiological process often imbued with powerful cultural and religious symbols. For men, it is a mysterious and sometimes frightening phenomenon—the shedding of blood without visible injury. For women, it has been a double-edged sword. Far too often, it has been used in misogynist ideologies as evidence of the defiling and ungodly nature of the female body, leading many s…
Untouchability - Taboos - Bibliography
The notion of taboo has a peculiar history: it was originally a Polynesian term referring to a ritual prohibition against contact with a thing, an animal, or a person. The term eventually became a widely discussed anthropological concept, and finally, in its last avatar, has been adopted by most languages to refer to something that is strictly and collectively forbidden. This wide recognition ste…
Uplift
Uplift is the process by which the earth's surface slowly rises either due to increasing upward force applied from below or decreasing downward force (weight) from above. During uplift, land, as well as the sea floor, rises. The outer shell of the earth, the crust, divides into moving sections called plates. Uplift, forming mountains and plateaus, usually results as these plates crash into …
Upwelling
Upwellings are a flow to the surface of deep, cold, nutrient-rich waters from greater depths in the ocean. The most extensive upwellings are associated with persistent coastal currents that draw surface water away from or along the coast to be replaced by a surface-ward flow of deeper waters. The most famous of these sorts of regional upwellings are found off the west coast of southern South Ameri…
Uranus - Observations From Earth, Results From The Flyby Of The Voyager 2 Spacecraft, Uranus's Magnetic Field - Discovery, Puck
Uranus is the seventh planet from the Sun. It has a large size (its diameter is almost four times that of Earth) and mass, low mean density, fairly rapid rotation, and well-developed ring (11 components) and satellite (15 members) systems. The planet has a strong magnetic field with a large tilt (58.6°) to its rotation axis and offset (0.3 Uranus radius) from its center. Analysis of the obs…
Urea - Urea And Metabolism, Urea And Industry
Urea is a white, crystalline solid also known as carbamide. It is highly soluble in water and is the major molecule used by mammals and amphibians as a means of excreting nitrogenous waste (which generally comes from proteins). It is used in making fertilizers (where it serves as source of nitrogen) and in cattle feed, where it also raises the nitrogen levels. Urea is also used in the manufacturin…
Urology
Urology is the branch of medicine that deals with the urinary tract in females and with the urogenital tract in males. In both sexes, the urinary tract consists of the kidneys, ureters, bladder, and urethra. In males, additional structures such as the prostate gland are included in the urogenital system. The problems with which a urologist deals tend to fall into three general categories: infectio…
Utilitarianism - Bibliography
Utilitarianism is the name of a group of ethical theories that judges the rightness of acts, choices, decisions, and policies by their consequences for human (and possibly animal) welfare. These theories have been widely influential among philosophers, economists, and political and social scientists, and, in the early twenty-first century, in the general area of applied or practical ethics. It wou…
Utopia - Expressions Of Utopianism, Postmodernism, Non-western Utopianism, Bibliography
The word utopia was coined by Thomas More (1478–1535) as the name of the island described in his Libellus vere aureus nec minus salutaris quam festivus de optimo reip[ublicae] statu, deq[ue] noua Insula Vtopia (1516). While More wrote in Latin, he based his new word on Greek. More combined topos (place or where) with u or ou (no or not) to create nowhere, but in "Six Lines on the Isl…
Vaccine
A vaccine is a medical preparation providing immunity from a vaccine specific disease. Vaccines generally consist of a weakened (attenuated) or killed antigens, associated with a particular disease that are capable of stimulating the body to make specific antibodies to that disease. Vaccines use a variety of different substances ranging from dead microorganisms to genetically engineered antigens t…
Vacuum
Vacuum is a term that describes conditions where the pressure is lower than that of the atmosphere. A sealed container is said to be "under vacuum" in this case whereas it is "pressurized" when the pressure is higher than atmosphere. In a vacuum, it becomes necessary to define pressure microscopically. This means that the pressure, or force per unit area, is determined …
Vacuum Tube - History
A vacuum tube is a hollow glass cylinder containing a positive electrode and a negative electrode between which is conducted in a full or partial vacuum. A grid between these electrodes controls the flow of electricity. The hollow cylinder of a vacuum tube contains a filament, typically tungsten coated with another metal. When the filament is sufficiently heated by an electric current, it emits el…
Valence
Valence refers to a number assigned to elements that reflects their ability to react with other elements and the type of reactions the element will undergo. The term valence is derived from the Latin word for strength and can reflect an element's strength or affinity for certain types of reactions. The electrons in an atom are located at different energy levels. The electrons in the highest…
Van Allen Belts - Discovery Of The Radiation Belts Of Earth, Description, Related Geophysical Effects, Radiation Belts Of Other Planets - Artificial radiation belts, Limitations on space flight, Two common misperceptions
Radiation belts are enormous populations of energetic, electrically charged particles—principally protons and electrons—trapped in the external magnetic field of a planet. Durable radiation belts exist at the planets Earth, Jupiter, Saturn, Uranus, and Neptune but not at Mercury, Venus, or Mars. Nine artificial radiation belts of Earth were produced during the period 1958-62 by the i…
Van der Waals Forces
Van der Waals forces are weak attractive forces between electrically neutral atoms or molecules. They are much weaker than the electrostatic forces which bind charged atoms or molecules (ions) of opposite sign or the covalent forces that bond neighboring atoms by sharing electrons. These forces develop because the rapid shifting of electrons within molecules causes some parts of the molecule to be…
Vapor Pressure - Definition
Vapor pressure is a force exerted by the gaseous phase of a two phase—gas/liquid or gas/solid system. All liquids and solids have vapor pressure at all temperatures except at absolute zero, -459°F (-273°C). The pressure of the vapor that is formed above its liquid or solid is called the vapor pressure. If a substance is in an enclosed place the two phase system will arrive to …
Variable - Characteristics Of A Variable, Variables In A Function
A variable is a mathematical symbol which is used to represent a member of a given set and is typically denoted by a letter such as x, y, or z. The idea of a variable, invented during the late sixteenth century, is characteristic of modern mathematics and was not widely used in ancient times. Since a variable reflects a quantity which can take on different values, its use has become a critical par…
Variable Stars
Most people regard the stars as constant and unchanging. A character in one of Shakespeare's plays refers to a friend "as constant as the Pole Star." While Shakespeare was probably referring to the constant position of the Pole Star, he did not know about the precession of the equinoxes. Or, if he was referring to the constant light of the Pole Star (Polaris), he was in error …
Variance
Variance is one of several measures of dispersion which are used to evaluate the spread of a distribution of numbers. Such measures are important because they provide ways of obtaining information about data sets without considering all of the elements of the data individually. To understand variance, one must first understand something about other measures of dispersion. One measure of dispersion…
Varicella Zoster Virus
Varicella zoster virus is a member of the alphaherpesvirus group and is the cause of both chickenpox (also known as varicella) and shingles (herpes zoster). The virus is surrounded by a covering, or envelope, that is made of lipid. As such, the envelope dissolves readily in solvents such as alcohol. Wiping surfaces with alcohol is thus an effective means of inactivating the virus and preventing sp…
Variola Virus
Variola virus (or variola major virus) is the virus that causes smallpox. The virus is one of the members of the poxvirus group (Family Poxviridae). The virus particle is brick shaped and contains a double strand of deoxyribonucleic acid. The variola virus is among the most dangerous of all the potential biological weapons. At the time of smallpox eradication approximately one third of patients di…
Vegetables - Vegetables Derived From Stems, Petioles, Or Foliage, Vegetables Derived From Fruits Or Flowers, Vegetables Derived From Below-ground Tissues - Eat your vegetables
The word vegetable is not scientifically defined by botanists. Rather, the plants and plant parts that are considered to be vegetables have been specified by a broad consensus among farmers, grocers, and consumers. In general, vegetables are plant tissues that are eaten as a substantial part of the main course of a meal. In contrast, fruits have a culinary definition as relatively sweet, often unc…
Veins
Veins are vessels designed to collect and return blood, including deoxygenated hemoglobin, from tissues to the heart. In humans, veins and the venous vascular system can be divided in to three separate systems depending on anatomical relationships and function. Initially, veins can be divided into systemic and pulmonary systems. The veins that drain the heart, comprising the coronary venous system…