284 VELOCITY OF ELECTRICITY. SECT. XXVIII. 



a certain limit; whence those lambent diffuse flashes of 

 lightning without thunder so frequent in warm summer 

 evenings." 



The velocity of electricity is so great, that the most 

 rapid motion which can be produced by art appears to 

 be actual rest when compared with it. A wheel re- 

 , volving with celerity sufficient to render its spokes invis- 

 \ ible, when illuminated by a flash of lightning, is seen for 

 : an instant with all its spokes distinct, as if it were in a 

 state of absolute repose ; because, however rapid the 

 \ rotation may be, the light has come and already ceased 

 before the wheel has had time to turn through a sensible 

 space. This beautiful experiment is due to Professor 

 Wheatstone, as well as the following variation of it, 

 which is not less striking : Since a sunbeam consists of 

 a mixture of blue, yellow, and red light, if a circular 

 piece of pasteboard be divided into three sectors, one of 

 which is painted blue, another yellow, and a third red, 

 it will appear to be white when revolving quickly, be- 

 cause of the rapidity with which the impressions of the 

 colors succeed each other on the retina. But the in- 

 stant it is illuminated by an electric spark, it seems to 

 stand still, and each color is as distinct as if it were at 

 rest. This transcendent speed of the electric fluid has 

 been ingeniously measured by Professor Wheatstone ; 

 and although his experiments are not far enough ad- 

 vanced to enable him to state its absolute celerity, he has 

 ascertained that it much surpasses the velocity of light. 

 In the horizontal diameter of a small disc fixed on the 

 wall of a darkened room are disposed six small brass 

 balls, well insulated from each other. An insulated 

 copper wire half a mile long is disjoined in its middle, 

 and also near its two extremities ; the six ends thus ob- 

 tained are connected with the six balls on the disc. 

 When an electric discharge is sent through the wire by 

 connecting its two extremities, one with the positive, 

 and the other with the negative coating of a Leyden 

 jar, three sparks are seen on the disc, apparently at the 

 same instant. At the distance of about ten feet, a small 

 revolving mirror is placed so as to reflect these three 

 sparks during its revolution. From the extreme velocity 

 of the electricity, it is clear, that if the three sparks bo 



