Fig. 59
If we wish to turn 30° in ½″, we take the angle of 30° and look within it for an arc of ½″. The arc of the right length and the right angle being found, it can be drawn free-hand or mechanically, by tracing or by the dividers. Using this meter, we are able to draw any curve or combination of curves, approximately; and we are able to describe and define a line, in its curvatures, so accurately that it can be produced according to the definition. Owing, however, to the difficulty of measuring the length of circular arcs accurately, we may find it simpler to define the circular arc by the length of its radius and the angle through which the radius passes when the arc is drawn.
Fig. 60
Here, for example, is a certain circular arc. It is perhaps best defined and described as the arc of a half inch radius and an angle of ninety degrees, or in writing, more briefly, rad. ½″ 90°. Regarding every curved line either as a circular arc or made up of a series of circular arcs, the curve may be defined and described by naming the arc or arcs of which it is composed, in the order in which they are to be drawn, and the attitude of the curve may be determined by starting from a certain tangent drawn in a certain direction. The direction of the tangent being given, the first arc takes the direction of the tangent, turning to the right of it or to the left.
Fig. 61
Here is a curve which is composed of four circular arcs to be drawn in the following order:—
Tangent up-right 45°, arc right radius 1″ 60°, arc left radius ⅓″ 90°, arc right radius ¾″ 180°.