1824.] Conductors of Lightning. 431 



When lightning strikes a house, it usually falls on the chim-* 

 neys, either from their being the most elevated parts, or because 

 they are lined with soot, which is a better conductor than dry 

 wood, stone or brick. The neighbourhood of the fire place is 

 consequently the most insecure spot in a room during a thunder 

 storm, where it is safer to station oneself in a corner opposite 

 the windows, at a distance from every article of iron or other 

 metal of any considerable size. 



Persons are often struck by lightning without being killed, 

 and others have been wholly saved from injury by silk dresses, 

 which serve to insulate the body, and prevent the access of the 

 electric matter. 



Practical Part. 



Details respecting the Construction of Paratonnerres. 



A paratonnerre is a metallic bar, ABCDEF (PI. XXXIV), 

 fig. 1, rising above a building, and descending, without any 

 breaks, to the ground, its lower end plunging into a well of 

 water, or a wet soil. The vertical part B A is called the stem, 

 and projects into the air above the roof, and the part of the bar 

 B C D E F which descends from the foot of the stem to the 

 soil, is called the conductor. 



Of the Stem. 



The stem B A is a square bar of iron, tapering from its base 

 to the summit, in form of a pyramid. For a height of from 20 

 to 30 feet, which is the mean length of the stems placed on large 

 buildings, the. base is about 2i inches square.* 



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

 the point of the stem would soon become blunt, to prevent 

 which a portion, A H, is cut off from the end, A B, fig. 2, about 

 20 inches in length, and replaced by a conical stem of brass or 

 copper, gilt at its extremity, or terminated by a small platina 

 needle, A G, two inches long.f The platina needle is soldered 

 with silver solder to the copper stem ; and to prevent its sepa- 

 rating from it, which might sometimes happen, notwithstanding 

 the solder, it is secured by a small collar of copper, as seen in 

 fig. 3. 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 to each 

 other, and is then screwed into the iron stem, and secured there 

 also by a steady pin (see C, fig. 4). If the gilding of the point 

 cannot easily be performed on the spot, nor the platina readily 

 obtained, they may both be dispensed with without any incon- 



• The best way of making a pyramidal bar is to weld together pieces of iron end to 

 end, about two feet long each, and of successively decreasing diameters. 



t Instead of the platina needle, one of standard silver may be substituted, composed 

 of nine parts of silver and one of copper. 



