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lower front edges of the cube Ce e'D, forming one line, which must also be a thick one. Practice among mechanical draughtsmen varying as to the appropriate use of a medium thickness of line in a drawing, it may be remarked in the case here presented that if "colour" were used to assist the eye in determining the form of the object represented in elevation by E in the diagram, the tint would be dark near the edge ev, and gradually lighten off as it approached the edge BD, although the latter would cast a shadow on any surface behind it. For this reason a medium thickness of line seems best suited to the position. As to the lining-in of the plan P of the cube, it will at once be seen from its position that only one edge, or that between points f and , will cast a shadow, and it must therefore be made a thick line. The other three two in the plane of the light and one having light falling direct upon it must be fine lines. 38. To prevent the misreading by the student of some engravings of mechanical subjects in outline still to be met with, it is as well here to observe that in England some draughtsmen and engravers line-in both plans and elevations of objects as if they were situated in the same plane, and cast their shadows in the same directions ; but that this is contrary to the principles upon which their projections were obtained may at once be seen from a study of the shadows cast by the cube in plan and elevation in Fig. 133. As the planes of projection are at right angles to each other, it would be impossible for the shadows to fall in the same direction in both planes, unless the light fell upon the objects represented in two totally different directions at one and the same time, which would be absurd. In all representations of objects, therefore, throughout this work, where any difference in the lining is