144 Miscellaneous Intelligfince. 



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 ])uildings 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. According, 

 however, to the opinion of Charles, a paratonnerre will effectually 

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

 the height of the conductor. By increasing, therefore, the altitude 

 of a conductor, the space also which it will protect is augmented 

 in proportion. 



A current of electric matter, whether luminous or not, is always 

 accompanied by heat, the intensity of which depends on the ve- 

 locity of the current. This heat is sufficient to make a metallic 

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

 hence we may perceive the absurdity of some attempts which have 

 been lately made, to protect ships, by thin slips of copper nailed 

 to their masts. The heat of the electric fluid scarcely raises the 

 temperature of a bar of metal, mi account of its large mass ; and 

 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 should 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, is sufficient for the conductor 

 of the paratonnerre. 



According to Gay-Lussac, a paratonnerre consists of two parts, 

 the stem which projects into the air above the roof, and the con~ 

 ductor, which descends from the foot of the stem to the ground. 

 The stem he proposes to be a square bar of iron, tapering from 

 its base to the summit, in form of a pyramid, and for a height of 

 from 20 to 30 feet, which is the mean length of the stems placed 

 on large buildings ; the base should be about 2| inches square. 

 Iron being very liable to rust by action of air and moisture, the 

 point of the stem would soon become blunt ; and therefore, to pre- 

 vent it, a portion of the top, about 20 inches in length, should be 

 composed of a conical stem of brass or copper, gilt at its extre- 

 mity, or terminated by a small platina needle, two inches long. 

 Instead of the platina needle, one of standard silver may be sub- 

 stituted, composed of nine parts of silver, and one of copper. The 

 platina needle should be soldered with a silver solder to the copper 

 stem ; and to prevent its separating from it, which might some- 

 times happen, notwithstanding the solder, it should be secured by 

 a small collar of copper. The copper stem is united to the iron 

 one, by means of a gudgeon, which screws into each ; the gudgeon 

 is first fixed in the copper stem by two steady pins at right angles 



