SECT, xxvin.] VELOCITY OF ELECTEICITY. 323 



record of an electric cloud of high tension being dispelled by 

 a conducting rod silently withdrawing the electric fluid ; yet 

 it may mitigate the stroke, or render it harmless if it should 

 come. Copper conductors afford the best protection against 

 lightning, especially if they expose a broad surface, since 

 the electric fluid is conveyed along the exterior of bodies. 

 Conductors do not attract the electric fluid from the clouds ; 

 their object is to carry it off in case of a stroke, and therefore 

 they ought to project very little, if at all, above the building. 



When the air is highly rarefied by heat, its coercive power 

 is diminished, so that the electric fluid escapes from the 

 clouds, and never can be accumulated beyond 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 revolving with celerity 

 suificient to render its spokes invisible, 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 striding: 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, because of the rapidity 

 with which the impressions of the colours succeed each other 

 on the retina. But, the instant it is illuminated by an 

 electric spark, it seems to stand still, and each colour 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 



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