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.
ELLIPSES.
An ellipse is a geometrical figure, and can be drawn as described in [geometrical problem 29], [page 96]; many drawing offices keep sets of hard rubber ellipses, to economize time constructing them.
Fig. 169.
Fig. 170.