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A circle rotating on a straight line remains in the strip of plane
        comprised between this line and its parallel at a distance equal
        to the diameter of the circle. As it continues to move, the circle is always
        in contact with both lines. It is a
 figure with constant amplitude.
 
Initially, it seems strange that there are other curves
        with the same qualities. However, it is easy to draw as many curves as possible.
    
   
 
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