232 WRITINGS OF JOSEPH HENRY. [1845 



is principally confined to the surface. From a series of ex- 

 periments presented to the American Philosophical Society, 

 by Professor Henry, on this subject, it appears that the elec- 

 trical discharge passes, or tends to pass, principally at the 

 surface ; and as an ordinary sized house is commonly fur- 

 nished with from two to four perpendicular gutters (gener- 

 ally two in front and two in the rear), the surface of these 

 will be sufficient to conduct silently, the most violent dis- 

 charge which may fall from the clouds. 



Professor Henry also stated that he had lately examined 

 a house struck by lightning, which exhibited some eflPects of 

 an interesting kind. The lightning struck the top of the 

 chimney, passed down the interior of the flue to a point op- 

 posite a mass of iron placed on the floor of the garret, where 

 it pierced the chimney; thence it passed explosively, (break- 

 ing the plaster,) into a bedroom below, where it came in con- 

 tact with a copper bell-wire, and passed along this horizon- 

 tally and silently for about six feet ; thence it leaped explosively 

 through the air a distance of about ten feet, through a dormer 

 window, breaking the sash, and scattering the fragments 

 across the street. It was evidently attracted to this point by 

 the upper end of a perpendicular gutter, which was near the 

 window. It passed silently down the gutter, exhibiting 

 scarcely any mark of its passage until it arrived at the term- 

 ination, about a foot from the ground. Here again an ex- 

 plosion appeared to have taken place, since the windows of 

 the cellar were broken. A bed in which a man was sleeping 

 at the time, was situated against the wall, immediately under 

 the bell-wire; and although his body was parallel to the 

 wire, and not distant from it more than four feet, he was not 

 only uninjured, but not sensibly affected. The size of the 

 hole in the chimney, and the fact that the lightning passed 

 along the copper wire without melting it, show that the dis- 

 charge was a small one, and yet the mechanical effects, in 

 breaking the plaster, and projecting the window frame across 

 the street, were astonishingly great. 



These effects the Professor attributes to a sudden repulsive 

 energy, or expansive force developed in the air along the 



