but equal velocities. When looked at from a particular position, they presented the appearance of a wheel with immovable radii.
When the two wheels of a gig or carriage in motion are looked at from an oblique position, so that the line of sight crosses the axle, the space through which the wheels overlap appears to be divided into a number of fixed curved lines, passing from the axle of one wheel to the axle of the other, in general form and arrangement resembling the lines described by iron filings between the opposite poles of a magnet. The effect may be obtained at pleasure by cutting two equal wheels out of white cardboard (Plate III. fig. 1), each having from twelve to twenty or thirty radii, sticking them on a large needle two or three inches apart, revolving them between the fingers, and looking at them in the right direction against a dark or black ground; the greater the velocity of the wheels the more perfect will be the appearance (fig. 2).
When the dark-coloured wheel of a carriage is moving on a good light-coloured rod, so that the sun shines almost directly on its roadside, and the wheel and it shadow are looked at obliquely, so that the one overlaps the other in part, then, in the overlapping part, luminous or light lines will be perceived curved more or less, and conjoining the axle and its shadow, if the wheel and shadow are superposed sufficiently; or, tending to do so, if they are superposed only in part: the more rapid the motion the more perfect is the appearance. The effect may be easily observed by making a pasteboard wheel like one of those just described, blackening it, sticking it on a pin, and revolving it in the sunshine, or in candlelight, before a sheet of white paper (fig. 3). If the wheel be converted into a tetotum or top, by having a pin thrust through its centre, and spun upon a sheet of white paper, the effect produced by the wheel and its shadow will be obtained with facility, and in form will resemble fig. 2. In all these cases no rims are required; the spokes or radii produce the effect.
If a carriage wheel running rapidly before upright bars, as a palisade or railing, be observed, the attention being fixed upon the wheel, peculiar stationary lines will appear: those perpendicular to the nave or axis will be straight, but the others curved; and the curve will be greatest in those which
