DIAMETER

(Gr. Ikx,, through, and f^ir^ov, a measure.) A straight line pa ing through the centre of a geometrical figure, as that of a circle, ellipse, or hyperbola. Many figures have diameters, though they have, properly speaking, no centres; in this case, the diameter is the straight line which bisects all the parallel chords. Some geometers extend this definition, and apply the term diameter to the curve lines which pa through the points of bisection of all the parallel chords of other curves. In this sense every curve has a diameter. In astronomy the apparent diameters of the celestial bodies are estimated by the number of seconds \a. the angles they subtend at the eye. The apparent diameter (which is measured by the micrometer), compared with the distance of the body, gives its real diameter; for the distance of the sun or a planet is to its real diameter, as radius to the sine of the angle subtended by tlie apparent diameter of the body. Diameter. In Architecture, the measure acro the lower part of the shaft of a cohimn, which is usually divided into sixty parts, called minutes, and forms a scale for the measurement of all the parts of an order. [s. 349]