nothing lurther upon the subject than what he has gained by reading the general instructions in Chapter II. In the succeeding lessons the more elementary instructions are not repeated, the notes being confined to new points arising out of the increased difficulties of the example!! Therefore it wil. be necessary, for a full comprehension of the subject, that the student shall work through the drawings in the order in which they are given. In no case should a drawing be copied to less than twice the size shown, and the more intricate ones should be made to still larger scale.
It will perhaps be advisable, before entering into the actual instructions for copying and producing drawings, to explain briefly, by aid of diagrams, the theory of right-angled projection, usually distinguished as orthographic. This term is derived from the Greek, " orthographia "—orthos, correct; grapho, to write—signifying correct writing. In those days the distinction between writing and drawing was not so marked as now, so the word has come to si and for " correctly drawn " in the sense that the drawing is correct as to its dimensions and shape. The word projection is derived from two Latin words, pre, forth, and jacio, to throw; so that the literal meaning of " orthographic projection " is a correct view thrown from the object upon a plane surface.
It must also be understood that the " projectors" thrown forth are at right angles or perpendicular to the plane of projection—-that is to say, the surface upon which the drawing is made. If otherwise, the resulting drawing might be a " projection,'' but it would not be an orthographic projection.
To understand this clearly, refer to Fig. i, page 47. This is a sketch of a triangular block of wood suspended in the air for a purpose that will presently be seen, and arranged with its several edges parallel to three planes which are mutually perpendicular to each other. The planes are lurther shown in dotted lines unfolded into one plane,
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