1824.] Conductors of Lightning. 429 



It sometimes leaves visible traces of its passage, even at a depth 

 of more than 30 feet. When also a paratonnerre has any breaks 

 in it, or is not in perfect communication with a moist soil, the 

 lightning, having struck it, flies from it to some neighbouring 

 body, or divides itself between the two, in order to pass more 

 rapidly into the earth. Frequent instances of serious accidents 

 have occurred from both these causes. 



Before the flash ensues, the influence of the thunder cloud 

 disturbs the natural electrical state of all the bodies below it 

 at the surface of the earth, and brings them into a state contrary 

 to its own; and thus every object becomes a centre of attraction 

 towards which the lightning has a tendency to direct itself. In 

 order that this effect may be suddenly produced, it is indispensa- 

 ble that the bodies influenced by the cloud be good conductors 

 and in perfect communication with a moist soil. 



A paratonnerre perfectly connected with the ground, and ter- 

 minating in a very sharp point instead of being rounded off, 

 may become so intensely electrified by -the influence of a thun- 

 der cloud, as to give off a continual stream of electric matter, 

 which sometimes is visible in the dark, appearing as a luminous 

 pencil at the extremity of the point, and must certainly tend, in 

 part at least, to neutralise the electrical state of the thunder 

 cloud. A rounded point may exert an equal, or even a greater 

 attraction on the thunder cloud than a sharp one ; but if the flow 

 of electric matter from the point become very rapid, the lightning 

 will strike sooner, and from a greater distance between the 

 cloud and the paratonnerre, than if its extremity were rounded ; 

 at least electrical experiments lead to this conclusion. 



Thus the most advantageous form that can be given to a 

 paratonnerre appears evidently to be that of a very sharp cone. 

 The higher a paratonnerre is elevated in the air, other circum- 

 stances being equal, the more its efficacy will be increased, as 

 is clearly proved by the experiments with electrical kites, made 

 by MM. de Romas and Charles. 



It has not been accurately ascertained how far the sphere of 

 action of a paratonnerre extends, but, in several instances, the 

 more remote parts of large buildings on which they have been 

 erected, have been struck by lightning at the distance of three 

 or four times the length of the conductor from the rod. It is 

 calculated by Charles, that a paratonnerre will effectually protect 

 from lightning a circular space, whose radius is twice that of the 

 height of the conductor ; and they are now attached to build- 

 ings after that rule. 



A current of electric matter whether luminous or not, is 

 always accompanied by heat, the intensity of which depends on 

 the velocity of the current. 'This heat is sufficient to make a 

 metallic wire red hot, or to fuse or disperse it, if sufficiently thin ; 

 but it scarcely raises the temperature of a bar of metal, on 



