- 行业: Printing & publishing
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- Company Profile:
McGraw Hill Financial, Inc. is an American publicly traded corporation headquartered in Rockefeller Center in New York City. Its primary areas of business are financial, publishing, and business services.
Chickenpox (varicella) and shingles (herpes zoster) are two different forms of disease caused by the varicella-zoster virus, which is a deoxyribonucleic acid (DNA) virus closely related to herpes simplex and Epstein-Barr viruses. Initial infection causes varicella, a common childhood infection characterized by fever, malaise, and a rash consisting of dozens to hundreds of small fluid-filled lesions (vesicles) that are individually surrounded by reddened skin. Successive crops of lesions appear that eventually ulcerate and crust over during the two-week course of the disease. The virus is spread from person to person by the highly infectious respiratory secretions and lesion drainage. Varicella is rarely a serious disease in normal children but can be severe in immunocompromised individuals or in the rare adult who escaped childhood infection. Primary infection results in immunity to a new varicella-zoster virus, but the original virus lies dormant in nerve ganglia cells.
Industry:Science
Diagnostic imaging techniques such as magnetic resonance imaging (MRI), ultrasound, and computerized tomography (CT) have had an enormous impact in clinical applications. Magnetic resonance imaging offers a noninvasive means to map brain structure and function by sampling the amount, flow, or environment of water protons in vivo. Intrinsic contrast can be augmented by the use of paramagnetic contrast agents in both clinical and experimental settings. However, these agents are little more than anatomical reporters which can at best label individual fluid compartments or distinguish tissues that are magnetically similar but histologically distinct. To permit a more direct imaging of the physiological state of cells or organs, an entirely new class of smart magnetic resonant imaging contrast agents has been developed that change their influence on the nearby water protons in a conditional fashion. The agents modulate fast water exchange, yielding distinct “on” and “off” states.
Industry:Science
From an orbiting or flyby spacecraft, only two kinds of measurements provide information about the interior of a planetary body: gravity and magnetic field measurements. With the exception of radar soundings of ice, all other remote sensing measurements from spacecraft provide information about the atmosphere or the surface to a depth not exceeding perhaps a few centimeters. The spacecraft trajectory is affected by gravity, which in turn reflects how mass is distributed in the body. However, the detection of a magnetic field of internal origin provides evidence that a dynamo is active in the interior. Planetary dynamos are driven by thermal convection of electrically conducting fluids in the core and lower mantle of the body. In the case of the Earth, the fluid is an iron-rich, molten magma. Rotation of a planet is believed to be an essential ingredient of the process, organizing the convection patterns into cells leading to the existence of a self-regenerating magnetic field.
Industry:Science
Globular clusters are gravitationally bound stellar systems containing, on average, about 100,000 stars. With one significant exception (omega Centauri), it appears that all stars in any one cluster formed at the same time out of a parental gas cloud, and all the stars began their lives with the same chemical composition. Not all globular clusters are alike, however. Some have very low abundances of elements heavier than helium, so that <i>Z</i>, the heavy-element mass fraction, may be over 100 times lower than that found in the Sun's atmosphere (where <i>Z</i> is about 0.018). Other clusters have relatively high <i>Z</i> values, reaching almost as high as that of the Sun. Most of the globular clusters in the Milky Way Galaxy lie within a few kiloparsecs of the galactic center, but some lie as far out as 100 kiloparsecs (1 kiloparsec equals 3260 light-years, 3.1 × 10<sup>16</sup> km, or 1.9 × 10<sup>19</sup> mi; for reference, the Sun lies about 8 kiloparsecs from the center).
Industry:Science
Bones provide structural support for ambulatory activities and protection for vital organs, such as the brain and heart. They also respond to changing concentrations of circulating hormones and locally released cytokines and growth factors, and along with cells in other organs regulate the blood concentration of calcium, an element essential for many critical functions, such as cell division and heart contraction. This regulatory activity takes precedence over the support function because bones can be weakened to the point of fracture following excessive removal of bone by osteoclasts (large multinucleated cells, associated with bone resorption) in response to estrogen or vitamin D deficiency. During embryonic development, the size, shape, position, and number of bones are determined by the activities of genes that are expressed at specific times by distinct groups of cells. Some of these genes also regulate the formation and activity of cells that remodel bones after they are formed.
Industry:Science
Autonomous navigation means that a vehicle is able to plan its path and execute its plan without human intervention. In some cases remote navigation aids are used in the planning process, while at other times the only information available to compute a path is based on input from sensors aboard the vehicle itself. An autonomous robot is one which not only can maintain its own stability as it moves but also can plan its movements. Autonomous robots use navigation aids when possible but can also rely on visual, auditory, and olfactory cues. Once basic position information is gathered in the form of triangulated signals or environmental perception, machine intelligence must be applied to translate some basic motivation (reason for leaving the present position) into a route and motion plan. This plan may have to accommodate the estimated or communicated intentions of other autonomous robots in order to prevent collisions, while considering the dynamics of the robot's own movement envelope.
Industry:Science
Black holes are compact massive objects from which light cannot escape. The largest of these are the supermassive black holes, which are millions to billions of times more massive than the Sun. Dormant supermassive black holes reside at the centers of nearly all present-day galaxies. Their presence has been detected through the orbital motions of stars and gas near the galactic centers. Locally the mass of a galaxy's supermassive black hole is known to be tightly related to the mass of its host galaxy. Given the very limited extent of a supermassive black hole's gravitational influence, this relation hints at a deep connection between the processes that formed the stars in the host galaxy and the processes that formed the central supermassive black hole. Astronomers would like to follow this connection back in time in order to understand the growth of supermassive black holes in galaxies, but this requires obtaining a census of all the accreting supermassive black holes in the universe.
Industry:Science
Black holes are the remnants after heavy stars burn out and collapse. The gravitational pull of a black hole is so strong that not even light can escape. Moreover, craters and other distinctive features are impossible because of the crushing forces exerted on matter in the black hole. The contrast between the initial complexity of a star and the final, apparently structureless state of a black hole is dramatic and puzzling. Indeed, if the black hole shows no hint of its complex past, there is an apparent conflict with determinism. According to determinism, the present state of the world developed from some past state that should be deducible from the present one, at least in principle. Theoretical physicists are strong believers in determinism and therefore are inclined to suspect that the complexity of the star has been transformed into some kind of internal structure of the black hole, rather than having disappeared. Recent research may have uncovered the appropriate hidden structure.
Industry:Science
Attempts to unify three fundamental interactions—strong, electromagnetic, and weak—with a postulate that the three forces, with the exception of gravity, can be unified into one at some very high energy. The basic idea is motivated by the incompleteness of the electroweak theory of S. Weinberg, A. Salam, and S. Glashow, which has been extremely successful in the energy region presently accessible with the use of accelerators, and by the observation that the coupling constant for strong nuclear forces becomes smaller as energy increases whereas the fine-structure constant (α = 1/137) for electromagnetic interactions is expected to increase with energy. The Weinberg-Salam-Glashow theory, which unifies electromagnetic and weak interactions, is incomplete in that strong interactions are not included; two coupling constants in the theory are unrelated; and in spite of many properties shared by them, quarks and leptons are unrelated, and their mass spectra and other properties remain unexplained.
Industry:Science
Devices that respond to incident ultraviolet, visible, or infrared electromagnetic radiation by giving rise to an output signal, usually electrical. Based upon the manner of their interaction with radiation, they fall into three categories. Photon detectors are those in which incident photons change the number of free carriers (electrons or holes) in a semiconductor (internal photoeffect) or cause the emission of free electrons from the surface of a metal or semiconductor (external photoeffect, photoemission). Thermal detectors respond to the temperature rise of the detecting material due to the absorption of radiation, by changing some property of the material such as its electrical resistance. Detectors based upon wave-interaction effects exploit the wavelike nature of electromagnetic radiation, for example by mixing the electric-field vectors of two coherent sources of radiation to generate sum and difference optical frequencies. This discussion concentrates on photon and thermal detectors.
Industry:Science
