PROJECTION—PLAN DRAWING.

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plan-drawing, and to descriptive Geometry .—I have men-tioned the method of constructing one figure equal to ano-ther by the use of parallel lines ; and the same means areemployed as a general method of representing or of descri-bing bodies. This is the object of designing in descriptivegeometry.

The object to be represented, is transferred to a plane,called the plane of projection, such as a table, a plank,an extended sheet of paper, &c. From every point of theobject itself, lines are drawn parallel to any direction,which may be chosen on account of its convenience. Itwill be readily conceived, that each point of the bodyrepresented may thus be transferred to the plane of pro-jection, by following the parallel direction which has beenchosen. The new position of the point on the plane isthe projection of the point.

If all the points of a right line, or of a curve, are thusprojected, they will form, on the plane of projection, anew right line or a curve, which will be the projection ofthe primitive right line or curve.

This species of projection, or design, is made use ofto represent objects in civil, military, or naval architec-ture, in carpentry, in sculpture, in making plans for ma-chines, &c.

A single representation of objects is not enough to de-termine exactly their form and size. For this purposewe must have two plans; and it is found convenient tosuppose one of them horizontal and the other vertical.To make the vertical plan, the object to be represented isprojected by parallel lines which are horizontal; to makethe horizontal plan, the object is projected by parallel lineswhich are vertical.

The horizontal projection is, properly speaking, the planof an object; the vertical projection is called the eleva-tion of an object.

The student should be thoroughly aware of the import-ance, and of the necessity of knowing and practising withgreat exactness the art of linear projection. He should be