I BELIEVE that Dr. Phipps is mistaken in his assertions (1) that the round trip velocity vr.t. of a point on the edge of a rotating disk, defined by vr.t. = Ï r = 2Ï r/T, where T is the period, is not equal to the instantaneous edge velocity, v, and (2) that Ï r can become greater than c, the velocity of light.