PROTECTION FROM LIGHTNING. 



103 



That paratormerres exert their protective power only when lightning strikes 

 the. structure over which they preside, is an error easily corrected, by immedi- 

 ate experiment, independently of the refutation it might receive on theoretical 

 grounds. Let the continuity of one of these apparatuses be broken, by sepa- 

 rating any two bars of the series, so that their ends, instead of being in imme- 

 diate contact, shall be distant by the eighth or tenth part of an inch from each 

 other. When stormy clouds pass over the apparatus, a continual stream of elec- 

 trical light will be visible in the interval between the separated points of the 

 bars. If their distance be increased to an inch, sparks will be observed to 

 pass between them, in rapid and continual succession, accompanied by deto- 

 nations as loud as the report of a pistol. 



Captain Wynne, who commanded a British frigate, lately observed, during a 

 storm, at a point where, by accident, an interruption of the metallic continuity 

 of his paratonnerre occurred, an almost unintermitting succession of sparks, 

 which continued for two hours and a half, the whole interval during which the 

 thunder-clouds were over the vessel. 



It is apparent, then, that paratonnerres are not merely instrumental in saving 

 a structure when lightning actually falls upon it, but they also possess a pre- 

 ventive power, and gradually and silently disarm the clouds by draining the 

 electric fluid from them ; and this process commences the moment the clouds 

 approach a position vertically over the paratonnerre. 



The explanation of these phenomena is easy, when the principles which 

 govern the movements of the electric fluids are understood. From the mo- 

 ment that a stormy cloud passes over a paratonnerre, and comes within the 

 range of its influence, the electricity of the cloud decomposes the natural elec- 

 tricities of the rod, attracting that of the contrary iwme, which is accordingly ac- 

 cumulated at the point, and repelling that of tie same name, which is driven 

 into the crust of the earth, or into the water with, which the lower extremity of 

 the paratonnerre is in communication. The electricity of the contrary name, 

 collected at the point, soon acquires so great a tension that it overcomes the 

 restraining pressure of the air, and escapes in a jet, which may often be seen 

 in the dark, in the form of a luminous aigrette, issuing from the metallic point. 

 The fluid which thus escapes, enters into combination with the fluid of a con- 

 trary name, with which the cloud is charged, and neutralizes it. 



On land, and especially in cities, numerous objects are presented to the elec- 

 tricity of the air, which have this tendency to neutralize it, and marked effects, 

 such as that now referred to, are of more rare occurrence j but at sea such ap- 

 pearances are common, as is proved by the familiarity of all seamen with the 

 fire of St. Elmo, Castor and Pollux, and Helen, already mentioned. Experi- 

 ence proves that, cateris paribus, the more elevated a paratonnerre is, the more 

 efficacious it will be. 



This is easily verified by immediate experiment. The influence of a para- 

 tonnerre, or what is the same, the rate at which it neutralizes the electricity of 

 the air, is estimated by the number of sparks which pass in a given time through 

 a space of a given length — suppose, for example, an inch — by which its me- 

 tallic continuity is broken. It is found, that, according as the elevation of the 

 point of the rod is increased, the number of sparks transmitted undergoes a corre- 

 sponding increase. The height of the point being preserved, the number of ! 

 sparks transmitted in a given time is diminished by bringing other pointed con- | 

 ductors near it, and still more so if these conductors are more elevated. i 



The increased efficacy obtained by augmenting the elevation of the metallic j 

 point of a paratonnerre, is strikingly illustrated by the experiments which the < 

 contemporaries and successors of Franklin made with kites. Romas, having j 

 elevated kites by means of cord lapped with metallic wire, like the base-strings < 



