This question of light and shadow is what expresses the surface formation of every drawing. Simple strokes form outlines of the object, but their thickness, and the shading, show the character enclosed by the lines.[p. 100] Direction of Light.—Now, as stated, the casting of the shadow downward from the upper left-hand corner makes the last line over which it passes the thickest, and in [Figs. 105] and [106] they are not the extreme lines at the bottom and at the right side, because of the close parallel lines.
In [Figs. 109] and [110] the blades superposed on the other are very thin, and the result is the lines at the right side and bottom are made much heavier.
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| Fig. 107. | Fig. 108. |
| Illustrating Heavy Lines[ToList] | |
This is more fully shown in [Figs. 107] and [108]. Notice the marked difference between the two figures, both of which show the same set of pulleys, and the last figure, by merely having the lower and the right-hand lines of each pulley heavy, changes the character of the representation, and tells much more clearly what the draughtsman sought to convey.
Scale Drawings.—All drawings are made to a[p. 101] scale where the article is large and cannot be indicated the exact size, using parts of an inch to represent inches; and parts of a foot to represent feet.
In order to reduce a drawing where a foot is the unit, it is always best to use one-and-a-half inches, or twelve-eighths of an inch, as the basis. In this way each eighth of an inch represents an inch. If the drawing should be made larger, then use three inches, and in that way each inch would be one-quarter of an inch.
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| Fig. 109. | Fig. 110. |
| Illustrating Heavy Lines[ToList] | |
The drawing should then have marked, in some conspicuous place, the scale, like the following: "Scale, 1 1/2" = 1'"; or, "Scale 3" = 1'."
Degree, and What it Means.—A degree is not[p. 102] a measurement. The word is used to designate an interval, a position, or an angle. Every circle has 360 degrees, and when a certain degree is mentioned, it means a certain angle from what is called a base line.