SKCT. XXVIII. THE BACK STROKE. 293 



eleetru-ity 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 equili- 

 brium will be instantly 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 stroke, which is often of inconceivable inten- 

 sity. Instances have occurred when large masses of iron and stone, 

 and even many feet of a stone wall, have been carried to a con- 

 siderable distance by a stroke of lightning. Rocks and the tops 

 of mountains often bear the marks of fusion from its intense 

 heat ; and occasionally vitreous tubes descending many feet into 

 batiks of sand mark its path. 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 Pade.rborn 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 lightning. 

 No doubt the soil and substrata must influence its direction, since 

 it is found by experience that places which have been once struck 

 by lightning are often struck again. An insulated conductor on 

 the approach of a storm 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 atmospheric electricity. Copper conductors afford 

 the best protection, especially if they expose a broad surface, 

 since electricity is conveyed along the surface of bodies. There 

 is no instance of an electric cloud of high tension being dispelled 

 by a conductor, yet those invented by Sir William Snow Harris, 

 and universally employed in the navy, afford a complete protec- 

 tion in the most imminent danger. The Shannon, a 50-gun 

 frigate, commanded by the brave and lamented Sir William Peel, 

 was enveloped in a thunder-storm when about 90 miles to the 

 north-west of Java. It began at fifty minutes past four in the 

 afternoon ; the ship was driven before the storm, in a high sea, 

 amid streams of vivid lightning, deafening thunder, hail, and 

 rain. At five o'clock an immense ball of fire covered the inain- 

 topgallant mast, ran up the royal pole, and exploded in the air 

 with a terrific concussion, covering all the surrounding space 



