bie to exhale vapor in such quantities as to produce a column of conducting 

 matter above them, and are, for this reason, often struck by lightning, when 

 not provided with the means of protection afforded by PARATONNERRES. 



It sometimes happens that lightning falling among a crowd selects an indi- 

 vidual through whose body it passes to the ground, neglecting the rest, and this 

 without any discoverable cause. 



A case has been already mentioned in which this occurred from the influence 

 of a mass of metal concealed behind the wall against which the person who 

 suffered stood. But cases are not wanting in which we are compelled to admit 

 that different individuals are endowed with the conducting power in different 

 degrees, and, therefore, that the lightning strikes by preference the best con- 

 ductor. The results of experiments with artificial electricity corroborate this, 

 for in transmitting the electric discharge through a chain of persons, it has 

 sometimes happened that one individual in the chain stops the fluid. From 

 some unknown peculiarity of his organization, his body is a non-conductor. If, 

 then, it be ascertained that in some, though very rare instances, individuals 

 are found who are non-conductors, analogy leads to the inference that different 

 individuals have the conducting quality in different degrees. 



The fear engendered by the proximity of the cloud in which lightning is 

 elaborated, is founded, not on any distinct and explicable principles, but on a 

 vague impression that the chances of damage are augmented as we approach 

 the cause of danger, whatever that cause may be. If, then, the risk of injury 

 be admitted to increase as the distance from the thunder-cloud is diminished, it 

 would follow, by necessary inference, that destruction would be inevitable to 

 those whose temerity or misfortune might place them actually within the dimen- 

 sions of the cloud. Experience, however, does not justify this. On the con- 

 trary, thunder-clouds have been repeatedly traversed with impunity. In August, 

 1770, the abbe Richard, passed through a thunder-cloud on the small mountain 

 called Boyer, between Chalons and Tournus. Before he entered the cloud the 

 thunder rolled as it is wont to do. When he was enveloped in it, he heard only 

 single claps with intervals of silence, without roll or reverberation. After he 

 passed above the cloud, the thunder rolled below him as before, and the lightning 

 flashed. 



The sister of M. Arago witnessed similar phenomena between the village of 

 Estagel and Limovx ; and the officers of engineers engaged in the trigonometri- 

 cal survey repeatedly experienced the same occurrences on the Pyrenees. 



The paratonnerres, appended to buildings and ships, consist of a pointed metal- 

 lic rod, attached to, and projecting upward from the highest point of the structure 

 placed under their protection. The lower end of this rod is connected with a 

 series of other metallic rods, or with a metallic chain, which is continued to 

 the ground. If the paratonnerre be applied to a building, the series of rods 

 being attached to the walls and carried to the ground, must be continued to such 

 a depth, and brought to such a position, that its inferior extremity shall either 

 be immersed in water, or in soil which is in a permanent state of moisture. 

 The water, or moist soil, possessing the conducting power, receives the elec- 

 tricity from the extremity of the rod without explosion ; but if the rod termi- 

 nated in dry earth the fluid would escape from the extremity, or worse still, 

 from some other part of the series of rods, with an explosion, and would dam- 

 age whatever bodies might be adjacent to it. If it be applied to a ship, the 

 pointed rod is attached to the point of the main-top-mast, and the lower end of 

 the rod is connected with a chain which is carried down the mast and rigging 

 over the side of the vessel, and finally plunged in the sea. The highest point 

 of the rod being liable to be heated by lightning, and to be oxydated, is formed 

 of platinum, or gilt, so as to restrict oxydation. 



