394 WRITINGS OF JOSEPH HENRY. [1855- 



discharge a cloud in order to prevent a rupture of the air, it 

 being only necessary to draw off a quantity of the fluid suf- 

 ficient to reduce it just below that which is required to pro- 

 duce the explosion ; and for this effect there may be required 

 but a very slight diminution in the intensity of a cloud 

 which is at about the striking distance, to prevent an explo- 

 sion, particularly when we consider the prodigious number 

 of sparks which during thunder-storms were silently with- 

 drawn from the cloud by the pointed rod erected by Beccaria, 



Arago has collected a large number of instances, from 

 which it appears that the erection of a rod lessened the num- 

 ber of the explosive discharges. 



The Campanile. of St. Marks, at Venice, from the multi- 

 tude of the pieces of iron in its construction, was in a high 

 degree exposed to danger from lightning, and in fact prior 

 to 1776, had been known to be struck nine times. In 

 the beginning of that year a conductor was placed upon it, 

 and since that time the edifice has been un-injured by light- 

 ning. 



Previous to 1777, the tower of Sienna was frequently 

 struck, and on every occasion much injured. In that year 

 it was provided with a conductor, and has since received 

 one discharge, but with no damage. 



In the case of a church at Carinthia, on an average four 

 or five strokes of lightning annually were discharged upon 

 the steeple until a conductor was erected, after which one 

 stroke was received in five years. At the Valentino palace 

 the lightning conductors established by Beccaria, caused the 

 entire disappearance of strokes of lightning which were pre- 

 viously of frequent occurrence. 



The monument in London, although only accidentally pro- 

 vided with a virtual conductor, appears to have been exempt 

 from damage by lightning for nearly one hundred and eighty 

 years. 



The action of the rod in diminishing the intensity of the 

 cloud however, can only be of a very temporary character, 

 and cannot, as some have supposed, affect its subsequent 

 state, or disarm it of its fulminating power, since its elec- 



