> 106 PROTECTION FROM LIGHTNING. 



v ought to be immersed in water or in wet soil. If is necessary to add, that if 

 s it be in water, an artificial cistern will not serve the purpose, as it is in general 

 ? stanch, and enclosed on every side by non-conductors of electricity. Exam- 

 S pies of the inefficiency of such a termination to the conductor are not wanting. 



< The cathedral of Milan was struck by lightning on the 9th of June, 1819, and 

 S the lighthouse at Genoa, on the 4th January, 1827, and in both cases damage 

 ( was sustained, notwithstanding the paratonnerres. On examination, it proved 

 j that the inferior extremities of these apparatus were immersed in artificial 



< cisterns. 



5 To increase the surface of contact of the conductor with the ground it has 



< been proposed to make it diverge into several points at its lower end, or to fiat- 

 J ten it into a thin broad plate. It has also been proposed to immerse it in a bed 

 c of charcoal, previously raised to a red-heat, this being a good conductor of 

 $ electricity. 



( When several paratonnerres are erected on the same building, each should 

 communicate with the ground by the nearest and most direct route, the fluid 

 by such means passing more freely through them. Their efficiency will be 

 still more augmented if they communicate with each other, and with all the 

 metallic parts of the roof. 



Flexible metallic wires combined together so as to form a metallic rope, such 

 as are sometimes used for suspension bridges, have been proposed as substi- 

 tutes for rigid bars in paratonnerres as being more capable of adapting them- 

 selves to the inequalities of buildings, and less liable to lose their metallic 

 continuity by the effects of rust. 



When iron beams or cramps are used in the construction of a building, they 

 are sometimes carefully separated from the paratonnerres by non-conductors, 

 such as resin or pitch. If the paratonnerres be properly constructed, this pre- 

 caution is unnecessary. The lightning will go to the earth in preference to 

 any lesser mass of conducting matter. 



In the adaptation of paratonnerres to powder-magazines, danger is supposed to 

 arise from the electric sparks, which issue at parts of the conductor where mi- 

 nute and imperceptible breaches of continuity may take place. The sparks, 

 catching the powder which may be accidentally scattered on the projecting 

 parts of the building, or lodged in crevices by the wind, may produce fatal ef- 

 fects. For this reason it has been proposed that the paratonnerres for such 

 structures should not be erected on the building, but that they should be planted 

 in the ground near it. In that case, the practical principle already explained, 

 by which the range of the protective influence of the conductor is limited, must 

 be attended to, and a sufficient number of paratonnerres be placed round the 

 building to defend every part of it. 



With the view to prove the efficacy of paratonnerres, independently of all 

 reasoning based on theory', M. Arago has collected a number of facts, which 

 are too interesting, and have too strong a bearing, to be passed without some 

 notice here. We shall, therefore, briefly state the most important of them. 



The temple at Jerusalem stood from the time of Solomon till the year 70 of 

 the Christian era, a period of above 1000 years. It was completely exposed 

 to the violent storms incidental to Palestine. It was never struck by lightning. 

 Neither the Bible nor Josephus mentions any such fact, which, if it had occur- 

 red, must have strongly excited attention, and certainly been recorded. Be- 

 sides, it was covered with wood both within and without, and must have been 

 set fire to if it had been struck. Michaelis rightly infers that, in the course of 

 ten centuries, in the midst of continual thunder-storms, and ages before the in- 

 vention of paratonnerres, this building was never struck by lightning. The cause 

 is easily explained. By a circumstance apparently fortuitous, the temple was 



