n in the opposite direction.

For screw threads of a large diameter it is not uncommon to draw in the
thread curves as they appear to the eye, and the method of doing this is
shown in Figure 208. The thread is first marked on both sides of the
bolt, as explained, and instead of drawing, straight across the bolt,
lines to represent the tops and bottoms of the thread, a template to
draw the curves by is required. To get these curves, two half-circles,
one equal in diameter to the top, and one equal to the bottom of the
thread, are drawn, as in Figure 208.

[Illustration: Fig. 208.]

These half-circles are divided into any convenient number of equal
divisions: thus in Figure 208, each has eight divisions, as _a_, _b_,
_c_, etc., for the outer, and _i_, _j_, _k_, etc., for the inner one.
The pitch of the thread is then divided off by vertical lines into as
many equal divisions as the half-circles are divided into, as by the
lines _a_, _b_, _c_, etc., to _o_. Of these, the seven from _a_, to _h_,
correspond to the seven from _a'_ to _g'_, and are for the top of the
thread, and the seven from _i_ to _o_ correspond to the seven on the
inner half-circle, as _i_, _j_, _k_, etc. Horizontal lines are then
drawn from the points of the division to meet the vertical lines of
division; thus the horizontal dotted line from _a'_ meets the vertical
line _a_, and where they meet, as at A, a dot is made. Where the dotted
line from _b'_ meets vertical line _b_, another dot is made, as at B,
and so on until the point G is found. A curve drawn to pass from the top
of the thread on one side of the bolt to the top of the other side, and
passing through these points, as from A to G, will be the curve for the
top of the thread, and from this curve a template may be made to mark
all the other thread-tops from, because manifestly all the tops of the
thread on the bolt will be alike.

For the bottoms of the thread, lines are similarly drawn, as from _i'_
to meet _i_, where dot I is marked. J is got from _j'_ and _j_, while K
is got from the intersection of _k'_ with _k_, and so on, the dots from
I to O being those through which a curve is drawn for the bottom of the
thread, and from this curve a template also may be made to mark all the
thread bottoms. We have in our example used eight points of division in
each half-circle, but either more or less points maybe used, the only
requisite being that the pitch of the thread must be divided into as
m