pin in the focus F, and
tie to it one end of a piece of twine. Place a tracing-point at J, pass
the twine around the tracing-point, bringing down along the square-blade
and fasten it at N, with the tracing-point kept against the edge of the
square and the twine kept taut; slide the square along the
straight-edge, and the tracing-point will mark the half J C of the
parabola. Turn the square over and repeat the operation to trace the
other half J D. This method corresponds to the method of drawing an
ellipse by the twine and pins, as already described.

[Illustration: Fig. 85.]

To draw a parabola by lines: Bisect the width A B in Figure 85, and
divide each half into any convenient number of equal divisions; and
through these points of division draw vertical lines, as 1, 2, 3, etc.
(in each half). Divide the height A D at one end and B E at the other
into as many equal divisions as the half of A B is divided into. From
the points of divisions 1, 2, 3, etc., on lines A D and B E, draw lines
pointing to C, and where these lines intersect the corresponding
vertical lines are points through which the curve may be drawn. Thus on
the side A D of the curve, the intersection of the two lines marked 1 is
a point in the curve; the intersection of the two lines marked 2 is
another point in the curve, and so on.

TO DRAW A HEART CAM.

[Illustration: Fig. 86.]

Draw the line A B, Figure 86, equal to the length of stroke required.
Divide it into any number of equal parts, and from C as a centre draw
circles through the points of division. Draw the outer circle and divide
its circumference into twice as many equal divisions as the line A B was
divided into. Draw radial lines from each point of division on the
circle, and the points of intersection of the radial lines with the
circles are points for the outline of the cam, and through these points
a curved line may be drawn giving the shape of the cam. It is obvious
that the greater the number of divisions on A B, the more points and the
more perfect the curve may be drawn.

CHAPTER IV.

_SHADOW LINES AND LINE SHADING._

SECTION LINING OR CROSS-HATCHING.

When the interior of a piece is to be shown as a piece cut in half, or
when a piece is broken away, as is done to make more of the parts show,
or show more clearly, the surface so broken away or cut off is
section-lined or cross-hatched; that is to say, diagonal lines are drawn
across it, and to distinguish one pi