IRREGULAR CURVE OR SCROLL.
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.
Figs. 160-166.
Fig. 167.
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.