bulk composition of Earth (whole Earth composition) Information on the composition of the Earth as a whole has been deduced from: (a) cosmochemical models, which assume that the compositions of all members of the solar system are related, and that the bulk composition of the Earth can be inferred from the abundances of non-volatile elements in the Sun and some primitive meteorites (see CHONDRITIC EARTH MODEL); and (b) geophysical evidence, e.g. seismic data, density determinations, and magnetic surveys. The three layers, core, mantle, and crust, of which the Earth is formed differ markedly in their composition. The core and mantle make up more than 99% of the Earth's mass but their compositions can be only inferred, unlike that of the crust (see CRUSTAL ABUNDANCE OF ELEMENTS). The density and magnetic field of the Earth, information from seismic surveys, and the existence of iron—nickel meteorites lead to the conclusion that the core is predominantly iron, with a small proportion of lower-density element(s). Nickel is largely excluded because it would make the core too dense. The nature of the light component is controversial, but may be sulphur, carbon, oxygen, silicon, and potassium. From seismic evidence the mantle appears to be composed of dunite, peridotite, and eclogite, rocks similar in composition to chondritic meteorites. The upper mantle is probably formed from dense silicates of iron and magnesium, with silicon and magnesium oxides becoming commoner with depth. Information regarding the bulk composition of the Earth is fundamental to resolving such questions as the relationship between the Earth and the Moon. Perhaps the most critical single feature of the Earth's bulk composition is its content of K, U, and Th, since the radioactive isotopes of these elements control radioactive heat production, and therefore the thermal and geologic history of the Earth. See also COSMIC ABUNDANCE OF ELEMENTS; METEORITIC ABUNDANCE OF ELEMENTS; and SOLAR ABUNDANCE OF ELEMENTS.

The Coast and Geodetic Survey

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Measuring the United States. Established in 1807 and known as the United States Coast Survey until 1880, the Coast and Geodetic Survey was primarily involved in geodesy, or geodetic measurement, the survey of large land areas in which mathematical corrections are made to take into account the curvature of the earth’s surface. In 1871, at the urging of Coast Survey supervisor Benjamin Peirce, Congress appropriated funds to make a “geodetic connection” between the two coasts. They used an astronomical method recently devised by American engineer Andrew Talcott for determining latitude and employed the electric telegraph for finding longitude—taking advantage of new technology to improve the accuracy of their measurements. (Old methods of calculating longitude were accurate to one thousand feet; with the electric telegraph the margin of error was only one hundred feet.) The survey proceeded slowly from east to west using triangulation. In this process, once the exact longitudes and latitudes of two places are determined and the exact distance between them is measured, the location of a third place can be determined by the angles that lines drawn from it make with a line drawn between the first two places.

Studying Gravity. During the 1880s the Coast Survey studied the acceleration of gravity to establish the degree to which it varied with latitude. Since the earth is not a perfect sphere, gravitational pull is greater at the North and South Poles, which are flattened, than it is at the equator. Thus, measuring gravity at various points on the globe could help geodesists to determine the actual shape of the earth and lead to greater accuracy of mapping. (Although the earth is now known to be somewhat pear-shaped, bulging in the Southern Hemisphere, scientists in the late nineteenth century theorized that it bulged at the equator.) The force of gravity was measured with a pendulum, which swings at slower and slower rates of speed the closer it gets to the equator. This project was mainly the work of Benjamin Peirce’s son, Charles Sanders Peirce. In 1880 he demonstrated to the Paris Academy of Sciences that his measurement of the acceleration of gravity was superior to those of distinguished French scientists. The worldwide reputation of the Coast and Geodetic Survey for gravity studies came to an end in 1891, when Charles Peirce resigned after arguing with the director, Thomas C. Mendenhall, over methods of measurement.

The Battle for Civilian Control. Throughout the last quarter of the century, the Coast and Geodetic Survey had to compete for funding and for the definition of its scientific mission with the U.S. Naval Hydrographic Office, established in 1866. The original distinction between the two government entities was that the Coast Survey would map the domestic shoreline while the naval office was responsible for mapping foreign coasts. In practice, however, there was considerable overlap, as naval officers were appointed to the Coast Service. The battle for civilian control of science occupied the Congress throughout the 1880s and resulted in the restriction of naval scientific efforts to oceanographie projects.

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Thomas G. Manning, U.S. Coast Survey vs. Naval Hydrographie Office A 19th-Century Rivalry in Science and Politics (Tuscaloosa & London: University of Alabama Press, 1988).

seismic survey The exploration of a subsurface geologic structure by means of seismic waves which are generated artificially. Reflection profiling is the most common surveying method, and refraction surveys are particularly important in land surveys for static corrections. Small seismic refraction surveys are commonly carried out for engineering geophysics site investigations.