Part 23 (1/2)
Experiments afterwards made on lightning obtained from the clouds by pointed rods, received into bottles, and subjected to every trial, have since proved this suspicion to be perfectly well founded; and that, whatever properties we find in electricity, are also the properties of lightning.
This matter of lightning, or of electricity, is an extreme subtle fluid, penetrating other bodies, and subsisting in them, equally diffused.
When, by any operation of art or nature, there happens to be a greater proportion of this fluid in one body than in another, the body which has most will communicate to that which has least, till the proportion becomes equal, provided the distance between them be not too great; or, if it be too great, till there be proper conductors to convey it from one to the other.
If the communication be through the air, without any conductor, a bright light is seen between the bodies, and a sound is heard. In small experiments, we call this light and sound the electric spark and snap; but in the great operations of nature, the light is what we call _lightning_, and the sound (produced at the same time, though generally arriving later at our ears than the light does in our eyes) is, with its echoes, called _thunder_.
If the communication of this fluid be by a conductor, it may be without either light or sound, the subtle fluid pa.s.sing in the substance of the conductor.
If the conductor be good, and of sufficient bigness, the fluid pa.s.ses through it without hurting it. If otherwise, it is damaged or destroyed.
All metals, and water, are good conductors. Other bodies may become conductors by having some quant.i.ty of water in them, as wood and other materials used in building, but not having much water in them, are not good conductors, and therefore are often damaged in the operation.
Gla.s.s, wax, silk, wool, hair, feathers, and even wood perfectly dry, are non-conductors: that is, they resist instead of facilitating the pa.s.sage of this subtle fluid.
When this fluid has an opportunity of pa.s.sing through two conductors, one good and sufficient, as of metal, the other not so good, it pa.s.ses in the best, and will follow in any direction.
The distance at which a body charged with this fluid will discharge itself suddenly, striking through the air into another body that is not charged, or not so highly charged, is different according to the quant.i.ty of the fluid, the dimensions and form of the bodies themselves, and the state of the air between them. This distance, whatever it happens to be between any two bodies, is called their striking _distance_, as, till they come within that distance of each other, no stroke will be made.
The clouds have often more of this fluid in proportion than the earth: in which case, as soon as they come near enough, (that is, within the striking distance,) or meet with a conductor, the fluid quits them and strikes into the earth. A cloud fully charged with this fluid, if so high as to be beyond the striking distance from the earth, pa.s.ses quietly without making noise or giving light, unless it meet with other clouds that have less.
Tall trees and lofty buildings, as the towers and spires of churches, become sometimes conductors between the clouds and the earth; but, not being good ones, that is, not conveying the fluid freely, they are often damaged.
Buildings that have their roofs covered with lead, or other metal, and spouts of metal continued from the roof into the ground to carry off the water, are never hurt by lightning, as, whenever it falls on such a building, it pa.s.ses in the metals and not in the walls.
When other buildings happen to be within the striking distance from such clouds, the fluid pa.s.ses in the walls, whether of wood, brick, or stone, quitting the wall only when it can find better conductors near them, as metal rods, bolts, and hinges of windows or doors, gilding on wainscot, or frames of pictures, the silvering on the backs of looking-gla.s.ses, the wires for bells, and the bodies of animals, so containing watery fluids. And in pa.s.sing through the house it follows the direction of these conductors, taking as many in its way as can a.s.sist in its pa.s.sage, whether in a straight or crooked line, leaping from one to the other, if not far distant from each other, only rending the wall in the s.p.a.ces where these partial good conductors are too distant from each other.
An iron rod being placed on the outside of a building, from the highest part continued down into the moist earth, in any direction, straight or crooked, following the form of the roof or other parts of the building, will receive the lightning at its upper end, attracting it so as to prevent its striking any other part; and, affording it a good conveyance into the earth, will prevent its damaging any part of the building.
A small quant.i.ty of metal is found able to conduct a quant.i.ty of this fluid. A wire no higher than a goose-quill has been known to conduct (with safety to the building, as far as the wire was continued) a quant.i.ty of lightning that did prodigious damage both above and below it; and probably larger rods are not necessary, though it is common in America to make them of half an inch, some three-quarters, or an inch, diameter.
The rod may be fastened to the wall, chimney, &c., with staples of iron. The lightning will not leave the rod (a good conductor) to pa.s.s into the wall (a bad conductor) through those staples. It would rather, if any were in the wall, pa.s.s out of it into the rod, to get more readily by that conductor into the earth.
If the building be very large and extensive, two or more rods may be placed in different parts, for greater security.
Small ragged parts of clouds, suspended in the air between the great body of clouds and the earth, (like leaf gold in electrical experiments,) often serve as partial conductors for the lightning, which proceeds from one of them to another, and by their help comes within the striking distance to the earth or a building. It therefore strikes, through those conductors, a building that would otherwise be out of the striking distance.
Long sharp points communicating with the earth, and presented to such parts of clouds, drawing silently from them the fluid they are charged with, they are then attracted to the cloud, and may leave the distance so great as to be beyond the reach of striking.
It is therefore that we elevate the upper end of the rod, six or eight feet above the highest part of the building, tapering it gradually to a fine sharp point, which is gilt, to prevent its rusting.
Thus the pointed rod either presents a stroke from the cloud, or if a stroke be made, conducts it to the earth, with safety to the building.
The lower end of the rod should enter the earth so deep as to come at the moist part, perhaps two or three feet; and if bent when under the surface, so as to go in a horizontal line six or eight feet from the wall, and then bent again downwards three or four feet, it will prevent damage to any of the stones of the foundation.