470 Mr. Snow Harris on Lightning Conductors 



of an air thermometer, and also by a similar rod with ragged 

 edges, placed near other metallic masses : the effect on the wire 

 remains unchanged *. 



It is not difficult to perceive the distinction of the two cases 

 just alluded to. If Dr. Priestley had insulated his wire, and 

 then charged it in the ordinary way, brushes of light would 

 doubtless have escaped from the angular portions; whereas 

 the wire when acting as a discharging circuit can exhibit no 

 such appearance. The electricity is then evanescent, and by 

 a law of electrical action determined rapidly toward the nega- 

 tive surface. Many facts might be adduced conclusive of this 

 point, but it seems scarcely worth while to dwell longer on it. 



16. The great end which the author proposes to himself in 

 this memoir, is an exposition of the danger attendant on my 

 method of fixed lightning conductors for ships, successfully 

 tried in the British navy for upwards of ten years; — with a 

 view to a substitution of an untried method of his own. It 

 may be worth while, therefore, in conclusion, to see whether 

 the objections he so strongly insists on, do not equally apply 

 to his own conductors as well as to mine, and, in short, to 

 lightning conductors generally. 



17. In the first place, he tells us (see 191.) " that it is pos- 

 sible for the most spacious conductor that can be applied to a 

 ship to be rendered sufficiently hot by lightning to ignite gun- 

 powder." 



18. In the next place, he says, (202.) that the " lateral dis- 

 charge will always take place when the vicinal bodies are ca- 

 pacious, and near the principal conductor or any of its me- 

 tallic appendages." This was the case, he says, when only 

 his small jar was used, and with this small jar he could pro- 

 duce lateral discharges at a distance of fifty " feet from the di- 

 rect discharge." 



19. Thirdly, he tells us (203.) that " the magnitude and 

 intensity of a flash of lightning being hifinitely greater than 

 anything which can be produced artificially, the lateral dis- 

 charges must be 'proportionally greater^" that is to say /«- 



Jinitely great. 



20. Taking these data as true then, it follows that any 

 lightning conductor carrying a flash of lightning, would at an 

 hifinite distance, produce a lateral explosion injinitely great, 

 and of course do an infinite deal of mischief. Hence, every 

 powder magazine having a lightning conductor, every ship 

 with a lightning chain in her rigging, should whenever light- 

 ning struck the conductor be destroyed ; for in no case is the 



* Fo.' a description of this instrument, termed an electro-thermometer, 

 see Transactions of the Royal Society for 1837, p. 18. 



