Irregular curves, or, as they are sometimes termed, sweeps, represented
by figs. 160-166, are used for curves that cannot be put in by the other
instruments. They are very useful when elliptical or parabolic curves
are desired, in preference to circles or arcs of a circle. They are much
used in design and architectural drawing. They are made of thin hardwood
or rubber, and sometimes of horn.
[Illustration: Figs. 160-166.]
[Illustration: Fig. 167.]
[Illustration: Fig. 168.]
Curves are irregular lines; a circle is a regular line. If a curve is to
be passed through a number of predetermined points it should be first
sketched in lightly, free-hand; a section of the scroll is then applied
to the curve so as to embrace as many points as possible; only the
central points of those thus embraced should be inked in; this process
is continued until the desired curve is completed.
Curves are made of various material, pearwood, cardboard, xylonite, hard
rubber, and a strip of soft lead is sometimes used, which may be easily
adjusted to the curve required.
The curves generally used in mechanical drawing are shown on previous
page.
Fig. 208, page 134, is a logarithmic _spiral_ curve. It is
mathematically constructed and contains every curve within the limit of
its size.