430 M. Gay-Lussac on [Dec. 



account of its large mass. It is by the heat of the electric 

 current, as well as by that disengaged from the air, condensed 

 by the passage of the lightning through it when not conveyed 

 by a good conductor, that buildings struck by it are frequently 

 set on fire. 



No instance has yet occurred of an iron bar, of rather more 

 than half an inch square, or of a cylinder of the same diameter, 

 having been fused, or even heated red hot by lightning. A 

 rod of this size would therefore be sufficient for a paratonnerre, 

 but as its stem must rise from 15 to 30 feet above the building, 

 it would not be of sufficient strength at the base to resist the 

 action of the wind, unless it were made much thicker at that 

 part. 



An iron bar, about three-quarters of an inch square, is suffi- 

 cient for the conductor of the paratonnerre. It might even be 

 made still smaller, and consist merely of a metallic wire, pro- 

 vided it be connected at the surface of the ground with a bar of 

 metal, about half an inch square, immersed in water or a moist 

 soil. The wire indeed would pretty certainly be dispersed by 

 the lightning, but it would direct it to the ground, and protect 

 the surrounding objects from the stroke. However, it is always 

 better to make the conductor so large as not to be destroyed by 

 the stroke, and the only motive for substituting a wire for a 

 stout bar is the saving in point of expence. 



The noise of the thunder generally occasions much alarm, 

 although the danger is then passed ; it is over indeed on the 

 appearance of the lightning, for any one struck by it neither 

 sees the flash, nor hears the clap. The noise is never heard till 

 after the flash, and its distance may be estimated at so many 

 times 368 yards, as there are seconds between the appearance of 

 the lightning and the sound of the thunder. 



Lightning often strikes solitary trees, because, rising to a 

 great height and burying their roots deep in the soil, they are 

 true paratonnerres, and their shelter is often fatal to the indivi- 

 duals who seek it ; for they do not convey the lightning with 

 sufficient rapidity to the ground, and are worse conductors than 

 men and animals. When the lightning reaches the foot of the 

 tree, it divides itself amongst the conductors that it finds near 

 it, or strikes some in preference to others, according to circum- 

 stances, and sometimes it has been known to kill every animal 

 that had sought shelter under the tree ; at others only a single 

 one out of many has perished by the stroke. 



A paratonnerre, on the contrary, well connected with the 

 ground, presents a certain security against the lightning, which 

 will never leave it to strike a person at its foot, though it would 

 not be prudent to station one self too close to it, for fear of some 

 accidental break in the conductor, or of its not being in perfect 

 communication with the ground. 



