An arc tangent to two others may encloseboth, or it may enclose only one and notthe other. In figure 4-38 the problem isto draw a circular arc with a radius equalto AB, tangent to and enclosing both arcsCD and EF. Set a compass to a radiusequal to AB less the radius of CD (indicatedby the dashed line from O), and, with Oas a center, strike an arc. Set the compassto a radius equal to AB less the radiusof EF (indicated by the dashed line fromO´), and, with O´ as a center, strike an inter-secting arc at P. The point of intersection ofthese two arcs is the center of a circle of whichan arc of given radius is tangent to, and encloses,both arcs CD and EF.In figure 4-39 the problem is to drawa circular arc with a radius equal to AB,tangent to, and enclosing, CD, and tangentto, but NOT enclosing, EF. Set a compassto a radius equal to AB less the radiusof arc CD (indicated by the dashed line from0), and, with O as a center, strike anarc, Set the compass to AB plus the radiusof EF (as indicated by the dashed line fromO´), and, with O´ as a center, strike an inter-secting arc at P. The point of intersection of thetwo arcs is the center of a circle of which an arcof the given radius is tangent to and encloses arcCD and also is tangent to, but does not enclose,arc EF.Figure 4-38.-Circular arc tangent toother circular arcs.and enclosing twoFigure 4-39.-Circular arc tangent to and enclosing one arcand tangent to, but not enclosing, another.Figure 4-40.-Curve composed of a series of consecutivetangent circular arcs.COMPOUND CURVESA curve that is made up of a series ofsuccessive tangent circular arcs is called acompound curve. In figure 4-40 the problem isto construct a compound curve passing throughgiven points A, B, C, D, and E. First, connectthe points by straight lines. The straight linebetween each pair of points constitutes the chordof the arc through the points.Erect a perpendicular bisector from AB. Selectan appropriate point O1 on the bisector as acenter, and draw the arc AB. From O1, draw the4-13
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