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conditions of position of the bounding edges of surfaces ; but of their use we shall speak later on. For the present we shall deal only with fine lines, and thick or shadow lines, and their application to plane- surfaced solids, or such as the student has already drawn-in in pencil. Knowing all the forms of the solids which have been taken as subjects for projection, their proper inking-in should present no difficulty. All that has to be determined in connection with the pencilled-in lines is what each represents that is to say, is it an edge or bounding line on which the light falls direct, or does it cast a shadow 1 This decided, the line can be drawn-in in ink. Taking the two projections of a cube given in the diagram Fig. 133 as an explanatory example let it be determined how they should be inked in. To begin with the elevation, and knowing the direction in which the light falls upon the object, it is evident that on two of its front edges AB and AD the light is falling direct ; their representa- tive lines AB and AD must therefore be fine ones. It is equally obvious that the edges BC and DC will cast a shadow on the YP against which the cube is resting, therefore the lines BC and DC, repre- senting these edges, must be thick, or shadow lines. Three other edges of the cube are also affected by the light viz., those of which A, B and D are the front ends ; but this cannot be indicated in the ele- vation. The cube whose plan is shown in the diagram is for explan- atory purposes set forward some distance from the IL, and must not MECHANICAL AND ENGINEERING DRAWING 8l therefore be taken as that of the cube given in elevation. In this case it is assumed to be standing with one of its faces on the HP. In that position two of its top edges cud' and a'b are directly in the