Periodic Center of Motion
Illustration showing the periodic center of motion as it often happens when two positions of a line are known and they are moving in the same plane and we wish to find an axis about which this line could revolve to occupy the two given positions.
Peter Schwamb, S.B. And Allyne L. Merrill, S.B. Elements of Mechanism (New York, NY: John Wiley & Sons, Inc., 1904)