Mathematical_Recreations-Kraitchik-2e

114 Mathematical Recreations
ways ring-radius intersections, and the required parallels are
easy to visualize. Now proceed, for any date, as follows:
1. Locate the Date and the Month on the outer ring; if
2,'-1&.23-:.., 3:::0~ ____ _
0",
FIGURE 18. Perpetual Calendar, Radius-ring Type.
they are two points, draw a line between them; if they coincide,
draw a tangent.
2. Locate the Century on the intermediate ring. Through
this point draw a line p'arallel to the line already drawn, until
it intersects the intermediate ring at another point; if this
line is tangent to the ring, then the starting point is also the