SECT. XXVIII. LIGHTNING CONDUCTORS. 283 



which large masses of iron and stone, and even many 

 feet of a stone wall, have been conveyed to a con- 

 siderable distance by a stroke of lightning. Rocks and 

 the tops of mountains often bear the marks of fusion 

 from its action; and occasionally vitreous tubes, de- 

 scending many feet into banks of sand, mark the path 

 of the electric fluid. Some years ago, Dr. Fiedler ex- 

 hibited several of these fulgorites in London, of con- 

 siderable length, which had been dug out of the sandy 

 plains of Silesia and Eastern Prussia. One found at 

 Paderborn was forty feet long. Their ramifications 

 generally terminate in pools or springs of water below 

 the sand, which are supposed to determine the course 

 of the electric fluid. No doubt the soil and substrata 

 must influence its direction, since it is found by experi- 

 ence that places which have been struck by lightning 

 are often struck again. A school-house in Lammer- 

 muir, East Lothian, has been struck three different 

 times. 



The atmosphere, at all times positively electric, be- 

 comes intensely so on the approach of rain, snow, wind, 

 hail, or sleet ; but it afterward varies, and the transi- 

 tions are very rapid on the approach of a thunder-storm. 

 An isolated conductor then gives out such quantities of 

 sparks that it is dangerous to approach it, as was fatally 

 experienced by Professor Richman, at Petersburg, who 

 was struck dead by a globe of fire from the extremity 

 of a conductor, while making experiments on atmos- 

 pheric electricity. There is no instance on record of an 

 electric cloud of high tension being dispelled by a con- 

 ducting rod silently withdrawing the electric fluid ; yet 

 it may mitigate the stroke, or render it harmless if it 

 should come. Copper conductors afford the best pro- 

 tection against lightning, especially if they expose a 

 broad surface, since the electric fluid is conveyed along 

 the exterior of bodies. Conductors do not attract the 

 electric fluid from the clouds ; their object is to carry 

 it off in case of a stroke, and therefore they ought to 

 project very little, if at all, above the building. 



When the air is highly rarefied by heat, its coercive 

 power is diminished so that the electric fluid escapes 

 from the clouds, and never can be accumulated beyond 



