GEOMETRY, DESCRIPTIVE, the name given to a branch of geometry, which has of late years been much cultivated by the French mathematicians, and in particular by Mongc, who may be regarded as its inventor. Its object is to represent on a plane, which has but two dimensions, any object which has three, and which admits of a strict definition. Descriptive geometry admits of a twofold ap plication. First, it is employed by artists to communicate to each other a knowledge of different objects. Thus it furnishes the means of constructing geographical and to pographical charts ; also plans of buildings and machines, architectural designs, sun-dials, theatrical decorations, &c. In this point of view, it is the best method that can be em ployed to describe the forms and the relative positions of objects. In the next place, it serves as an instrument of research, by which we may discover every thing relative to the form and the -position of the various parts of objects which admit of a rigorous definition. It is by the princi ples of descriptive geometry, that stone-cutters, carpen ters, ship-builders, and other artists, find the dimensions of the different parts of the works which they execute, in as far as these dimensions result from the complete definition of the object.
Descriptive geometry formed an essential branch of the education of the French youth in the school of public works established at the beginning of the revolution ; and it appears from the journal of the Polytechnic school, that the scholars were, during a certain period of the course, employed six hours every day in tracing the numerous ob jects which were the subject of their studies. The lessons given in the Normal School, from a treatise on the subject by Monge, entitled Geomeirie Descriptive, printed in 1799. There is also a treatise by Lacroix, entitled Essais de geo metric sur les plans et les surfaces conrbes (ou Elenzens de geometric descriptive.) We have already treated this sub ject under the head of CONSTRUCTIVE CARPENTRY. See CARPENTRY, Part II. ()