DIAMETER

, of a circle, is a right line passing through the centre, and terminated at the circumference on both sides.

The diameter divides the circumference, and the area of the circle, into two equal parts. And hals the diameter, or the semi-diameter, is called the radius.

For the proportion between the diameter and the circumference of a circle, see Circle and CIRCUMFERENCE.

Diameter of a Conic Section, or Transverse Diameter, is a right-line passing through the centre of the section, or the middle of the axis.—The diameter bisects all ordinates, or lines drawn parallel to the tangent at its vertex. See Conic Sections.

Conjugate Diameter, is a diameter, in Conic Sections, parallel to the ordinates of another diameter, called the transverse; or parallel to the tangent at the vertex of this other.

, of any Curve, is a right line which divides two other parallel right lines, in such manner that, in each of them, all the segments or ordinates on one side, between the diameter and different points of the curve, are equal to all those on the other side. This is Newton's sense of a Diameter.

But, according to some, a diameter is that line, whether right or curved, which bisects all the parallels drawn from one point to another of a curve. So that in this way every curve will have a diameter; aud hence the curves of the 2d order, have, all of them, either a right-lined diameter, or else the curves of some one of the conic sections for diameters. And many geometrical curves of the higher orders, may also have for diameters, curves of more inferior orders.

Diameter of Gravity, is a right line passing through the centre of gravity.

Diameter in Astromony. The diameters of the heavenly bodies are either apparent, i. e. such as they appear to the eye; or real, i. e. such as they are in themselves.

The apparent diameters are best measured with a micrometer, and are estimated by the measure of the angle they subtend at the eye. These are different in different circumstances and parts of the orbits, or according to the various distances of the luminary; being in the inverse ratio of the distance.

The sun's vertical diameter is found by taking the height of the upper and lower edge of his disk, when he is in the meridian, or near it; correcting the altitude of each edge on account of refraction and parallax; then the difference between the true altitudes of the two, is the true apparent diameter sought. Or the apparent diameter may be determined by observing, with a good clock, the time which the sun's disc takes in passing over the meridian: and here, when the sun is in or near the equator, the following proportion may be used; viz, as the time between the sun's leaving the meridian and returning to it again, is to 360 degrees, so is the time of the sun's passing over the meridian, to the number of minutes and seconds of a degree contained in his apparent diameter: but when the sun is in a parallel at some distance from the equator, his diameter measures a greater number of minutes and seconds in that parallel than it would do in a great circle, and takes up proportionally more time in passing over the meridian; in which case say, as radius is to the cosine of the sun's declination, so is the time of the sun's passing the meridian reduced to minutes and seconds of a degree, to the ar<*> of a great circle which measures the sun's apparent horizontal diameter. See Transit.

The sun's apparent diameter may also be taken by the projection of his image in a dark room.

There are several ways of finding the apparent diameters of the planets: but the most certain method is that with the micrometer.

The following is a table of the apparent diameters of the sun and planets, in different circumstances, and as determined by different astronomers.

The Mean Apparent diameters of the planets, as seen from the sun, are as follow:

For the true diameters of the sun and planets, and their proportions to each other, see Planets, SEMIDIAMETER, and Solar System.

Diameter of a Column, is its thickness just above the base. From this the module is taken, which measures all the other parts of the column.

Diameter of the Diminution, is that taken at the top of the shaft.

Diameter of the Swelling, is that taken at the height of one third from the base.