_Optical Query._--Last summer the following illusion was pointed out to me
at Sandwich, Kent. The ingenious horizontal machine to enable the treadmill
to grind the wind, in default of more substantial matter, although
certainly revolving only in one direction, say from right to left, at
intervals appeared to change its direction and turn from left to right.
This change appeared to several persons to take place at the same time, and
did not seem to be owing to any shifting of the perpendicular shutters for
regulating the resistance of the air. The point from which I viewed it was
near the south door of St. Clement's Church. Have any of the readers of "N.
& Q." noticed a similar illusion, and can they explain it?