SECT. XXVIII. VELOCITY OF ELECTRICITY. 285 



simultaneous, they will be reflected, and will vanish be- 

 fore the mirror has sensibly changed its position, how- 

 ever rapid its rotation may be, and they will be seen in a 

 straight line. But if the three sparks be not simultane- 

 ously transmitted to the disc if one, for example, be later 

 than the other two- the mirror will have time to revolve 

 through an indefinitely small arc in the interval between 

 the reflection of the two sparks and that of the single 

 one. However, the only indication of this small motion 

 of the mirror will be, that the single spark will not be 

 reflected in the same straight line with the other two, 

 but a little above or below it, for the reflection of all 

 three will still be, apparently simultaneous, the time in- 

 tervening being much too short to be appreciated. 



Since the number of revolutions which the revolving 

 mirror makes in a second are known, and the angular 

 deviation of the reflection of the single spark from the 

 reflection of the other two can be measured, the time 

 elapsed between their consecutive reflections can be as- 

 certained. And as the length of that part of the wire 

 through which the electricity has passed is given, its ve- 

 locity may be found. 



Since the number of pulses in a second requisite to 

 produce a musical note of any pitch is known, the num- 

 ber 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 mir- 

 ror which he employed in his experiments to make 800 

 revolutions in a second; and as the angular velocity of 

 the reflected image in a revolving 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 connection of the 

 physical sciences. 



When the atmosphere is highly charged with elec- 

 tricity, it not unfrequentiy happens that electric light in 

 the form of a star is seen on the topmast and yard-arms 



