$2 OMl LUMINOUS PHENOMEl^A. 



The electric that the electric explosion consists of a ball or cylinder of 



^^r or re ^^ great length, ignited by the compression of the air or 



gas, or other fluid it drives before it. Admitting this, the 



zigzag spark with ramifications may be considered as a fire 



ball continually throwing out detached pieces ; the brush 



will be a fire ball broken to pieces, and the lightning will 



not ditfer from fire balls but in its vicinity to the Earth, and 



its velocity, which is perhaps greater. An artificial fireball 



moving slowly has been seen once, and but once, by Warl- 



tire the lecturer. See Priestley's Electricity. 



The vfilocitjr The magnificent experiments of Watson on Shooter' s-hill, 



of disengaged ■ ^irhich the shock was transmitted through great lengths of 



electricity 23 , . _ ... 



miles in a se- wire, teach us nothing of the velocity of disengaged elec- 



nlrhT ^""^ ' tricity, as there is no proof that it has any known relation 

 to that of the electric matter passing through conductors. 

 .Rlost persons think they can distinguish the direction of light- 

 ning, but this may perhaps be a deception. M. Marat* is 

 the only philosopher that I know of, who has made any ob- 

 servation from which an niference of the velocity of light- 

 ning may be deduced ; and he himself remarks, that it is 

 attended with various causes of uncertainty. He measured 

 the angular distance between two clouds, from one of which 

 a horizontal flash of lightning fiew to the other, and found 

 ' it 30 degrees: the time was 20 tlilrds, and the distance de- 



termined from the interval of time between the flash and the 

 report, was 10,000 toises. From these data he infers, that 

 the velocity was 19,200 toises per second, which is somewhat 

 more than 23 English miles, 

 n-.dalsofora This determination, by its remarkable coincidence with 

 ' that of Sir Charles Blagden, respecting the velocity of fire 



balls, might lead to a conclusion, that there is a settled ve- 

 locity for luminous electric matter, if it were not credibly 

 but it is not ascertained, that it sometimes moves much slower, and is 

 likely to be even nearly stationary, according to circumstances. In the 

 constant. storm which happened at Steeple Ashtonf, on the 20th of 



June, 1772, two gentlemen being sitting in a parlour at the 

 vicarage-house, and conversing about a loud clap of thun- 



* Marat, Recherches physiques sur rEloctrlcite, p. 22G. 

 t Ph. Trans, to), C5, p. 2r.L'. 



der 



