LIGHTNING, PROTECTION FROM. When a flash of lightning strikes any object on the ground it generally does some destruction, apparently due to the fact that when the eleetrie discharge passes through a poor conductor it heats that substance intensely. Thus when a discharge passes through a living tree all the sap may be converted into vapor and all the passages filled with air or filled with sap burst open. When the flash runs along a metallic water-pipe or gas-pipe it heats and melts the solder at the junctions. When the flash strikes a shingle roof it sets fire to the wood. or. striking the chimney of a house, it ex pands the air within the brickwork or evaporates the water in the cracks, and thus breaks up the chimney by the explosion. When the discharge runs along the roots of a tree or an underground drain it tears up the ground by the force of the explosive expansion of the water.
In general, perfect protection from a destruc tive flash can only be obtained by surrounding an object with a continuous metallic covering. This need not necessarily be a metal box, but may consist of a network of wire, or even a tow metallic rods properly arranged: thus for an ordinary building the rods should run vertically up and down each of the corners and horizontally along the ridge, the eaves, and every edge or prominence on the roof; these are the ex posed portions. and if they are well protected there is but slight chance that a flash will strike the building elsewhere. A partial protection is of course provided for by a single metallic wire or so-called lightning-•od, especially if the upper end protrudes far above the building, but it is quite deceptive to suppose that any definite rule can be offered for calculating the so-called area of protection.
The conductors usually found on buildings have as their terminals sharp cones or prongs of copper which are gilded to protect the points and to prevent corrosion, while the conductors them selves are generally in the form of a metal rod, rope. or ribbon, and terminate either in plates buried in damp ground or by making connection with water or gas pipes. Modern authorities agree that the more points along the ridges and eaves of a roof, the greater the amount of safety, and these of course should point to the sky. A fac tory chimney or steeple in the neighborhood is apt to attract the lightning and in all cases should tie protected by conductors. as it may be a source of danger to surrounding buildings and not a safeguard as is commonly supposed. The lightning-conduetor should be of iron or copper. and if the latter material is employed, it is con sidered advantageous to use it in the form of tape weighing about six ounces to the foot. Iron,
however, answers as well. being preferred hy some authorities, and can he used in the shape of rod or tape with a weight of about thirty-five ounces to the foot. The lightnimi-conductor should be of the same material and continuous throughout, joints being either avoided or constructed most carefully. In its course to the earth care should be taken not to have sharp bends, corners. or curves. An important consideration is the con nection with the ground, and while it is possible to connect directly with water-pipes. an inde pendent ground is far better: running water, a deep well or boring, can be utilized with great advantage as a terminal for the conductor. A ton of charcoal or coke buried in the ground and a trench filled with the same material and leading to water also make a good ground.
In the United States there are lightning flashes so intense that a good lightning-rod, well earthed, will not provide an easy path for the lightning; and while such protection is not neces for houses in the city built in close blocks with metallic roofs and cornices, yet for barns and dwellings in the country authorities unite in pronouncing good lightning-conducto•s eminently desirable.
The Washington Monument at Washington.
D. C., has furnished one of the best instances of the effectiveness of lightning-conductors as a protective device. As it stands 555 feet in height in the centre of flat and well-watered ground. it should be a conspicuous mark for lightning. It is protected by means of four cop per rod, attached to the aluminum cap of the py'r:unidion at the top, and connecting kith the uprights supporting the elevator machinery. These copper rods are connected to each other by additional copper rods, and every live feet along the system there are copper points, num bering in all about 200. In the fifteen years elapsing since this improved arrangement was introduced, the monument has not been injured in any way by lightning. Peet when the aluminum point was less effectively connect•d with the vertical supports, there was one light ning stroke, which, while causing hut slight dam age to the cap, nevertheless oceasioned the pro vision of additional conductors. Dr. Oliver Lodge, the celebrated English authority on the subject of lightning-conductors. who has within a few year: advanced some novel ideas on this matter which have tended to change previous theories, says, "Almost any conductor is probably better than none, but few or no conductors are abso lute and complete safeguard." For further in formation, consult: Lodge. Lightning Conduct 01'S and Lightning Guards (Loudon, IS:12) ; and tuited States Weather Bureau bulletins on lightning.