spirit level, tool for determining whether a surface is horizontal. It consists essentially of a slightly bent transparent tube that is held in a frame. The tube contains some alcohol, ether, or similar fluid but is not entirely filled, so that it also contains a small bubble. The position of the bubble within the tube indicates whether the instrument is horizontal. The spirit level used by carpenters and masons has two tubes at right angles to each other, so that the device registers a vertical as well as a horizontal position. The plumb line was used to determine vertical position from ancient times until the 19th cent. At that time the spirit level, which was invented in France during the 17th cent., came into general use and replaced the plumb line in some applications.

sea level, the level of the sea, which serves as the datum used for measurement of land elevations and ocean depths. Theoretically, one would expect sea level to be a fixed and permanent horizontal surface on the face of the earth, and as a starting approximation, this is true. However, a number of factors operate to cause variations in sea level ranging up to several meters from place to place and to cause long-term global variations, often severe enough to cause flooding and damage to coastal zones. Sea levels vary greatly from one location to another, i.e., between Nova Scotia and Florida sea-level heights differ at about 16 in (40 cm). Locally the levels of the surface of the world's oceans are disturbed by wind-driven waves and tides. Sea level therefore fluctuates in periods ranging from seconds to a year as a result of these factors. Thus for some purposes it is necessary to know the mean sea level (MSL) in a particular area, determined by averaging the elevations of the sea's surface as measured by mechanical tide gauges over long periods of time. A number of other factors result in sea-level differences between one place or time and another. These may complement or counteract one another to result in a net rise or fall in mean sea level at a particular time and place. These factors include water temperature and salinity, air pressure, change of season, the amount of runoff from streams, and the amount of water stored as ice or snow on land. Characteristics of the earth also cause differences in sea level. Satellite measurements of the gravity field have shown that the earth is not a perfectly smooth sphere, making defining sea-level measurements difficult. Worldwide, or eustatic, sea levels have changed over time, such as the last ice age when sea level was as much as 333 ft (100 m) lower in many areas than today. These changes are due to a multitude of reasons. Past transgression (or rise) or regression (or lowering) of the seas have been caused mainly by the addition or removal of water from continental ice caps. Such sea-level fluctuations may be due to changes in climate that cause the ice to melt or accumulate. Theories of climate change include differences in carbon dioxide or oxygen levels, volcanic activity, or a change in the sun's energy. The increase in carbon dioxide caused by the burning of fossil fuel and deforestation of tropical rain forests may one day increase the overall temperatures on the earth, thus causing a rise in sea level. When the sun's rays warm the earth, carbon dioxide and other greenhouse gases trap the radiation, causing a "greenhouse effect" and global warming.

level: see spirit level.

Position of the air-sea boundary, to which all terrestrial elevations and submarine depths are referred. The sea level at any location changes constantly with changes in tides, atmospheric pressure, and wind conditions. Longer-term changes are influenced by changes in the Earth's climates. Consequently, the level is better defined as mean sea level, the height of the sea surface averaged over all stages of the tide over a long period of time.

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Device for establishing a horizontal plane. It consists of a small, sealed glass tube containing liquid and an air bubble; the tube is fixed horizontally in a block or frame with a smooth lower surface. When the bubble is in the middle of the glass tube, the device is on a level surface; adjustment to the horizontal is indicated by movement of the bubble. The glass tube is slightly bowed, and the level's sensitivity is proportional to the radius of curvature.

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