cbq

I haven't been able to confirm the actual rotation speed when the experiment was performed, but it is possible to get a rough idea using the applicable formula for estimating the speed of light, which is: c = da/dt x 2h/a, where h is the distance between the mirrors (the 20 miles you noted) and a is the angle the rotating mirror traverses while light travels this distance. The experimenter would measure an angle of deflection of the mirror that is twice a to come up with an estimate of the speed of light. Knowing the speed of light, the speed of mirror rotation can be calculated for various values of the deflection angle. The formula can be restated as da/dt = a x c/2h, or da/dt = a x 186,000 mi/sec / 40mi, or da/dt = 4650 x a degrees/sec. If the experimenter measured an angle of deflection of 4 degrees, then a=2, and da/dt = 9300 degrees/sec, or approximately 26 revolutions per second! See: http://en.wikipedia.org/wiki/Fizeau%E2%80%93Foucault_apparatus