322 BACK STROKE. [SECT, xxvur. 



called the back stroke. Suppose that the two extremities of 

 a cloud highly charged with electricity hang down towards 

 the earth ; they will repel the electricity from the earth's 

 surface, if it be of the same kind with their own, and will 

 attract the other kind ; and, if a discharge should suddenly 

 take place at one end of the cloud, the equilibrium will 

 instantly be restored by a flash at that point of the earth 

 which is under the other. Though the back stroke is often 

 sufficiently powerful to destroy life, it is never so terrible in 

 its effects as the direct shock, which is frequently of incon- 

 ceivable intensity. Instances have occurred in which large 

 masses of iron and stone, and even many feet of a stone wall, 

 have been conveyed to a considerable 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, descending many feet into banks of sand, mark the path 

 of the electric fluid. Some years ago, Dr. Fiedler exhibited 

 several of these fulgorites in London, of considerable 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 experience 

 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, becomes 

 intensely so on the approach of rain, snow, wind, hail, or 

 sleet ; but it afterwards varies, and the transitions are very 

 rapid on the approach of a thunder-storm. An isolated con- 

 ductor then gives out such quantities of sparks that it is dan- 

 gerous 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 experi- 

 ments on atmospheric electricity. There is no instance on 



