It follows that a line from B to M will always be tangential to the epicycloid.
Suppose b a tracing point on b, then as b rolls on a it will describe the epicycloid a b.
It is impossible to mill out even a convex cycloid or epicycloid, by the means and in the manner above described.
But if a circle be made to roll along the circumference of another circle, it becomes an epicycloid (which see).
The epicycloid shown is termed the “three-cusped epicycloid” or the “epicycloid of Cremona.”