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in the exposition of our subject, it will be necessary at this stage to define the meaning of the geometrical terms that will be made use of as we proceed, and to show /now, by a special combination of lines, those geometrical figures are constructed which form the surfaces of objects whose delineations are subsequently to be obtained by orthographic projection. The term "geometry," in its generally-accepted sense, means the science or knowledge of magnitude reduced to system, and has to do with the measurement of lines, surfaces, and solids. It has, like other sciences, two sides or branches, one " theoretical," which demonstrates or proves its principles, and the other " practical," or that which applies those principles to construction. Theoretical geometry, or Euclid, will seldom be referred to in the course of this work, as most of the demonstrations used are self-evident; but practical geometry a sub-divison of which is the basis of our subject must be understood by the student to such an extent as will enable him to work out the problems that will arise in the exposition of it. The two parts into which practical geometry is divided are : Plane geometry, which has reference only to the solution of questions relating to points, lines, and figures, situated in one plane ; and solid geometry, which shows by special representations on two or more defined planes, the relations of the points, lines, and surfaces of bodies having length, breadth, and thickness. We would, in passing, guard the student, on his entering on the study of geometrical drawing, against w r asting valuable time in working out the problems many of which will be of no use to him given in most text-books on the two subjects of plane and