512 



THUNDER-STORMS. 



of luminous undulations, which were derived from observations on 

 y<r;i:-lmbbles blown from a tobacco-pipe, it is found in the plaything of a child. 

 Every one knows that if the end of a lighted stick be whirled rapidly rounu in 

 a circle or other curve, it will present the appearance of a continued line of j 

 liL'iit, the lighted end, which occupies, in succession, every point of the curve, 

 appearing to the eye to be continually present at all its points. 



Ficr. 1. 



To develope the principle on which this fact rests, let fig. 1 represent a 

 wheel with ten thin spokes or radii, dividing its circumference in ten eqv.2,1 

 parts, and of some strong bright color, such as red. Let this wheel be put in 

 communication with clock-work, so as to be made to revolve uniformly at any 

 required rate. This wheel, having its face vertical, and turning on a horizon- 1 

 tal axis ; let a screen be placed before it, so as to conceal it from view, and in 

 this screen let an oblong opening be made, corresponding in magnitude and 

 position to that spoke of the wheel which is in the vertical position and pre- 

 sented from the centre upward. Let the screen, with such an aperture, be 

 represented in fig. 2. 



Fig. 2. 



As the wheel revolves its spokes pass the opening o, in succession, and if ) 

 the motion of the wheel be not very rapid, a person placed before the screen ( 

 will perceive the spokes appear and disappear in regular and uniform succes- ) 

 sion at the opening. If the velocity of the wheel be gradually increased, the > 

 succession of appearances and disappearances will be rendered, by degrees, ? 



^o 



