338 WRITINGS OF JOSEPH HENRY. [1855- 



mendous energy is exerted. In the case of a house examined 

 by the writer, the discharge fell upon the top of a chimney 

 at the west end of the building and passing through a stove- 

 pipe hole traversed the space under the rafters, (called the 

 cock-loft), to the chimney at the east end and thence down 

 to the ground ; the force exerted was sufficiently great to 

 lift up the whole roof from the top of the walls on which it 

 rested. In like manner, when the discharge takes place 

 along the upright timbers of a house, the clap-boards are 

 frequentl}'- blown off oiitward and the plaster inward as if 

 by the explosion of gunpowder. 



To a similar action we must ascribe the splintering of trees 

 by liglitning. At the moment of the passage of the dis- 

 charge the sap or moisture is suddenly endowed with a re- 

 pulsive energy which resembles in its effects the action of 

 an explosive compound, separating the fibres longitudinally 

 and projecting parts of the body of the tree to a distance. 

 When a tree is struck by lightning the greatest effect is 

 usually produced on the main stem just below the branches. 

 A portion of the discharge appears to be received on each 

 twig, leaf, and branch, and the whole concentrated by con- 

 verging towards the trunk. The repulsion imparted to the 

 atoms of a conductor is in some cases sufficiently great to 

 at once dissipate in vapor fine metallic wires, and this so 

 instantaneously that the silk covering by which they are 

 surrounded for telegraphic purposes is not burned. 



The repulsive energy is exerted not alone laterally, but 

 perhaps in a greater degree in the line of direction of the con- 

 ductor, tending to separate it as it were by transverse sec- 

 tions. Hence when electricity passes through a wall into 

 the interior of a house, a pyramidal mass of plaster is thrown 

 out. A similar effect is frequently produced when the dis- 

 charge takes place between the cloud and the level earth: a 

 large conical or pyramidal hole is formed, from which the 

 earth is thrown out as if by the explosion of a quantity of 

 powder beneath the surface. Such excavations are sup- 

 posed by some to indicate a discharge of electricity from the 

 earth to the cloud, but no conclusion of this kind can, with 



