PLAIN. A term used in contradistinction to mountain, though far from implying a level surface, and it may be either elevated or low.
PLAN. The area or imaginary surface defined by, or within any described lines. In ship-building, the plan of elevation, commonly called the sheer-draught, is a side-plan of the ship. (See [Horizontal Plan] and [Body-plan], or plan of projection.)
PLANE. In a general sense, a perfectly level surface; but it is a term used by shipwrights, implying the area or imaginary surface contained within any particular outlines, as the plane of elevation, or sheer-draught, &c.
PLANE-CHART. One constructed on the supposition of the earth's being an extended plane, and therefore but little in request.
PLANE OF THE MERIDIAN. See [Meridian].
PLANE-SAILING. That part of navigation which treats a ship's course as an angle, and the distance, difference of latitude, and easting or westing, as the sides of a right-angled triangle. The easting or westing is called departure. To convert this into difference of longitude, parallel, middle latitude, or Mercator's sailing is needed, depending on circumstances. Plane-sailing is so simple that it is colloquially used to express anything so easy that it is impossible to make a mistake.
PLANE TRIANGLE. One contained by three right lines.
PLANETS, Primary. Those beautiful opaque bodies which revolve about the sun as a centre, in nearly circular orbits. (See [Inferior], [Minor], and [Superior].)
PLANETS, Secondary. The satellites, or moons, revolving about some of the primary planets—the moon being our satellite.
PLANIMETRY. The mensuration of plane surfaces.