SECT, xxvrn.] VELOCITY OF ELECTRICITY. 325 



Since the number of pulses in a second, requisite to pro- 

 duce a musical note of any pitch, is known, the number of 

 revolutions accomplished by the mirror in a given time may 

 be determined from the musical note produced by a tooth or 

 peg, in its axis of rotation, striking against a card, or from 

 the notes of a siren attached to the axis. It was thus that 

 Professor Wheatstone found the mirror which he employed 

 in his experiments to make 800 revolutions in a second ; 

 and, as the angular velocity of the reflected image in a re- 

 volving mirror is double that of the mirror itself, an angular 

 deviation of one degree in the appearance of the two sparks 

 would indicate an interval of the 576,000th of a second ; 

 the deviation of half a degree would, therefore, indicate 

 more than the millionth of a second. The use of sound as 

 a measure of velocity is a happy illustration of the con- 

 nexion of the physical sciences. 



When the atmosphere is highly charged with electricity, 

 it not unfrequently happens that electric light, in the form 

 of a star, is seen on the topmast and yard-arms of ships. 

 In 1831 the French officers at Algiers were surprised to see 

 brushes of light on the heads of their comrades, and at the 

 points of their fingers when they held up their hands. This 

 phenomenon was well known to the ancients, who reckoned 

 it a lucky omen. 



Many substances, in decaying, emit light, which is attri- 

 buted to electricity, such as fish and rotten wood. Oyster 

 shells, and a variety of minerals, become phosphorescent at 

 certain temperatures when exposed to electric shocks or 

 friction : indeed, most of the causes which disturb molecular 

 equilibrium give rise to phosphoric phenomena. The 

 minerals possessing this property are generally coloured or 

 imperfectly transparent ; and, though the colour of this 

 light varies in different substances, it has no fixed relation 

 to the colour of the mineral. An intense heat entirely 

 destroys this property, and the phosphorescent light de- 

 veloped by heat has no connexion with light produced by 



