206 Mr. Faraday on a 



quently moved the two wheels in opposite directions with great 

 but equal velocities. When looked at from a particular posi- 

 tion, 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 over- 

 lap 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 form arid 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 (Fig. 1. Plate 3.), 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 road, so that the sun shines almost directly 

 on its broadside, and the wheel and its 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 de- 

 scribed, blackening it, sticking it on a pin, and revolving it in 

 the sunshine, or in candle light, 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 sha- 

 dow 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 perpen- 



