104 



PROTECTION FROM LIGHTNING. 



of a harp or violin, drew from the lower extremity of the cord flashes of light- 

 ning from three to four yards long, and an inch in thickness, accompanied by a 

 report as loud as that of a gun. It was remarked on several occasions that 

 thunder and lightning ceased when the fire was thus drawn from the cord. By 

 the same expedient thunder-clouds were drained of their fire, and converted 

 into common clouds, by Dr. Lining, of Charleston, and M. Charles. 



M. Arago proposes this expedient for averting the calamitous effects of hail- 

 stones which are so great a scourge to the agriculturist in several parts of 

 France. As the formation of hail is undoubtedly an effect of the sudden dis- 

 turbance of the electric equilibrium of the clouds, if the electric fluid could be 

 quietly and gradually drawn away, hail would be altogether prevented. Cap- 

 tive balloons might be substituted with advantage for kites, since they could 

 be elevated in a calm, and maintained at any required height. By such means 

 a multitude of experimental researches in electro-meteorology could be prosecu- 

 ted. The atmosphere could be sounded and the clouds themselves searched, 

 and their electrical contents submitted to careful and deliberate examination. 



The contest respecting pointed and blunt conductors, which was maintained 

 about the middle of the last century, has been already noticed. Although the 

 electrical laws, which have since then been so fully and clearly established, 

 can leave no doubt as to that question, an experiment decisive of it made by 

 Beccaria may be mentioned here. This philosopher placed on the roof of 

 San Gtovanni-di-Dio at Turin, a bar of iron, at the lower part of which was 

 such an interruption of continuity as to produce sparks when electricity passed 

 along it. The metallic point at the top was moveable on a joint, and con- 

 nected with a silken coid, by drawing which the observer could at pleasure 

 convert it into a blunt conductor, or restore to it the pointed form. In a 

 storm, so long as the point vas presented upward, a stream of sparks was 

 seen at the place where the breach of continuity was provided, but the moment 

 it was converted into a blunt conductor, the sparks either disappeared alto- 

 gether (which generally happened), o* passed in much less rapid succession. 



An ingenious calculation of the quantity of lightning drawn from the clouds 

 by paratonnerres, has been made by M. Ara^o. He states that in an ordinary 

 storm a hundred sparks would be transmitted through a small break of con- 

 tinuity in the conductor of which the combined effect would be sufficient to 

 kill a man, and these would pass in ten seconds.. As much lightning would 

 therefore pass per minute as would destroy six meii> as much per hour as 

 would kill three hundred and sixty men. He calculates in this way that the 

 paratonnerres erected by Beccaria on the palace of Valentino, combined with 

 the effects of the pointed parts of the roof, must take as much lightning per 

 hour from the clouds as would be sufficient to destroy three thousand men. 



The quantity of electricity which pointed conductors neutralize, may be im- 

 agined from the following circumstance : The British frigate Dryad, provided 

 with a paratonnerre (constructed according to the method proposed by Mr. 

 Snow Harris, by fixing to the mast itself narrow plates of thin copper), was 

 several times exposed to violent tornadoes off the coast of Africa. The elec- 

 tric fluid was seen on every part of these copper plates in such quantity as to 

 produce around them a sort of luminous atmosphere, accompanied by a noise 

 like that of water boiling violently. 



In the practical adaptation of paratonnerres, the determination of the range 

 of their protective influence is a problem of great importance. The physical 

 section of the Academy of Sciences at Paris, being consulted by the minister 

 of war on this point in 1823, adopted the estimate of M. Charles, and assumed 

 that a circle of double the height of the rod would be protected. 



If this estimate be interpreted with geometrical rigor, it would appear that 



