1824.] Conductors of Lightning. 437 



A building of 120 feet by the same rule, would require a stem 

 of 30 feet, and such are sometimes used ; but it is better, instead 

 of one stem of that length, to erect two of 15 or 18 feet, one 

 placed at 30 feet from one end of the building, the other at the 

 same distance from the other end, and consequently 60 feet 

 apart from each other, fig. 18. The same rule should be fol- 

 lowed for three, or any greater number of paratonnerres. 



For churches with steeples, although the paratonnerre on the 

 latter must from its great height extend its influence to a con. 

 siderable distance, yet as nothing decisive is at present known 

 from experiment as to the greatest distance to which it may 

 extend, it will be prudent to consider it as only protecting a 

 space, whose radius is equal to the height of its stem above the 

 ridge of the roof, and to erect other paratonnerres, on the roof 

 itself, according to the rule already given (see figs. 19 and 20). 



General Disposition of the Conductors of Paratonnerres. 



Although the necessity of establishing a very intimate com- 

 munication between the paratonnerre and the soil has already 

 been repeatedly insisted on. its importance is such that it may 

 be well to revert once more to the subject. If this condition be 

 not rigorously observed, the instrument will not only become 

 much less efficacious, but even dangerous, by attracting the 

 lightning without being able to convey it to the ground. What 

 other conditions remain to be stated are less important, but 

 nevertheless deserve attention. 



The lightning should always be conducted by the shortest 

 possible road from the stem of the paratonnerre to the ground. 



Agreeably to this principle, when two paratonnerres are 

 placed on a building, and terminate in one common conductor 

 (which is quite sufficient), the point from which its branches 

 diverge to the two stems, should lie evenly and at equal dist- 

 ances on the roof between them ; the common conductor and 

 its branches may be formed of an iron bar, of the same dimen 

 sions as for a single paratonnerre (see figs. 18 and 19). 



If there be three paratonnerres on a building, it will be pru- 

 dent to give them two conductors, fig. 20. In general each pair 

 of paratonnerres requires one conductor. 



Whatever number of paratonnerres be placed on a building, 

 they should all be connected together by establishing an intimate 

 communication between the feet of all their stems, by means of 

 iron bars of the same dimensions as those of the conductors, 

 figs. 20, 21,22. 



When the situation will admit of it, the conductor should be 

 placed on the wall of the building most exposed to the ruin, 

 which, by wetting it, renders it, though imperfectly, a conduc- 

 tor, and if the conductor of the paratonnerre be not in intimate 

 communication with the ground, it is possible that the lightning 



