Lightning, the illuminating flash produced by the discharge of atmospheric electricity, either between two clouds, or between a cloud and the earth, usually accompanied by a noise called thunder. It manifests itself in various forms, which have been called forked, zigzag, ball, sheet, and heat lightning. Zigzag lightning is produced by the discharge of a large quantity of electricity from a cloud through a resisting medium, which becomes compressed at various points and thus turns the current aside. As shown in the article Electeicity, experiments demonstrate that the more air is rarefied, other things being equal, the more readily will it permit the passage of the electric current, and the more it is compressed the more resistance it offers. Ball lightning has the appearance of a ball of fire, accompanied by a terrible explosion, and results from an unusually intense charge of electricity which forces a direct path. It has been supposed by some that the ball is an agglomeration of particles of ponderable matter which have been carried along with the current. Sheet lightning has the appearance of a diffuse glare of light, sometimes illuminating the edges and sometimes the whole surfaces of clouds.

It may be caused by a stroke of zigzag lightning at a great distance, sending its light through great thicknesses of clouds, so as to give it the appearance of diffuseness; or it may result from the passage of electricity of no great tension from particle to particle, like that produced in discharging an electrical machine over the surface of a bedewed pane of glass. Heat lightning differs but little from sheet lightning, being produced in the same two ways. When produced by the reflection or transmission of zigzag lightning, thunder is not heard on account of the distance. The color of lightning varies like that of the spark of the electrical machine when passed through the receiver of an air pump in various states of exhaustion, or when it contains different gases and vapors. - Of the nature of lightning the ancients knew nothing. Its disastrous effects were associated rather with the terrific sound of the thunder than with the flash, and the Greeks and Romans attributed them to the thunderbolt hurled by Jupiter to the earth. The Hebrews often represented them as direct exhibitions of divine power, and frequently in the Old Testament, as in Job xxxvii., the thunder is spoken of as the voice of the Lord. Even the earlier electricians did not suspect the identity of lightning and electricity.

The abbe Nollet in 1746 first drew attention to the similarity of effects exhibited by thunder clouds and the prime conductor of an electrical machine. Winckler next argued that the principle of the powers of each was identical. Franklin established the fact first by enumerating in a clear and methodical manner the various points of resemblance, and the similar effects produced by each, and finally by actually conducting the lightning to the earth in his well known experiment with the kite in Philadelphia. The following quotations from letters of Franklin written in 1749-'50, and contained in the " Observations on Electricity" published in London in 1869, are full of interest: "Where there is a great heat on the land in a particular region (the sun having shone on it perhaps for several days while the surrounding countries have been screened by clouds), the lower air is rarefied and rises; the cooler, denser air above it descends; the clouds in the air meet from all sides and join over the heated place, and if some are electrified, others not, lightning and thunder succeed and showers fall.

Hence thunder gusts after heats, and cool air after gusts. ... As electrical clouds pass over a country, high hills and high trees, lofty towers, spires, and masts of ships, chimneys, etc, as so many prominent points draw the electrical fire, and the whole cloud discharges there. Dangerous is it, therefore, to take shelter under a tree during a thunder gust. It is safer to be in the open fields for another reason. When the clothes are wet, if a flash in its way to the ground should strike your head, it may run in the water over the surface of your body, whereas if your clothes were dry it would go through the body. . . . Sulphurous and inflammable vapors arising from the earth are easily kindled by lightning. Besides what arise from the earth, such vapors are sent out by stacks of hay, corn, or other vegetables which heat and reek. . . . Now if the fire of electricity and that of lightning be the same, as I have endeavored to show in a former paper, and a tube of only 10 ft. long will discharge its fire at two or three inches distance, an electrified cloud of perhaps 10,000 acres may strike and discharge on the earth at a proportionally greater distance. ... I say if these things are so [speaking of the discharging power of points], may not the knowledge of this power of points be of use to mankind in preserving houses, churches, ships, etc, from the stroke of lightning, by directing us to fix on the highest parts of those edifices upright rods of iron made sharp as a needle, and gilt to prevent rusting, and from the foot of the rods a wire down the outside of the building into the ground, or down round one of the shrouds of a ship, and down her side till it reaches the water ? Would not the pointed rods probably draw the electric fire silently out of the cloud before it came near enough to strike, and thereby secure us from the most sudden and terrible mischief ? " It was not till three years afterward that Franklin actually made the experiment of drawing electricity from the clouds, and demonstrating the identity of atmospheric lightning and frictional electricity.

He had proposed various experiments, such as the erection of tall rods on the tops of spires. Dalibard in France, acting according to the instructions of Franklin, on May 10, 1752, obtained electrical sparks from an iron rod 40 ft. high in the garden at Marly, and charged Leyden jars from the same source. Franklin did not make his experiment with the kite till the 15th of June of the same year. These experiments were regarded with the highest interest by scientific men, and were repeated with various modifications in different parts of Europe. Prof. Richman of St. Petersburg, July 26 (Aug. 0), 1753, while explaining to a companion the construction of an electrometer attached to his conductor, was struck and instantly killed by what appeared to be a ball of blue fire as large as a man's fist, that was seen to leap from the insulated conductor to his head, a space of about a foot. A red mark was left on his forehead, his shoe was burst open, and his clothing slightly singed. His companion was benumbed and rendered senseless, and the door case and door were torn apart by the shock.

M. Romas, to whom the French academy of sciences awarded the merit of inventing the electrical kite more than a year before it was employed by Franklin, constructed a kite 7 ft. 5 in. high, and 3 ft. in its greatest width, with a surface of 18 sq. ft, A copper wire was wrapped around the string to increase its conducting power, and this was made to terminate in an insulating silk cord, near which an iron tube was placed to receive the electricity. The kite being raised to a height of 550 ft, on the approach of a storm, the iron conductor became so highly charged that electrical sparks were obtained, and shocks of great violence. As the storm increased, flashes of fire darted to the earth accompanied with explosions, and straws that happened to be on the ground were attracted alternately by the string and the ground, their movements being accompanied by electrical flashes and constant explosions. Such were the experiments by which the electrical nature of lightning was established, and the thunder proved to be the noise which accompanies the electrical discharge.

This sound may be prolonged as it is reflected in echoes by the clouds; or, as suggested by Sir John Herschel, it may come in successive impulses to the ear, as brought from an instantaneous discharge that extends for miles along a line directed away from the observer. So the terrific sudden crash may be the result of a flash occurring all round the observer with no great difference of distance from him in the points of the discharge. Not only was the electrical condition of the atmosphere during thunder storms thus established, but in 1753 the abbe Mazeas, by means of a wire 370 ft. long attached to a steeple at Maintenon, proved that electrical action is excited in clear, dry, and especially hot weather, at all hours between sunrise and sunset. - From a multitude of observations made by Cavallo, Read, De Saussure, and others, it appears that the atmosphere is almost always positively electrified in relation to the surface of the earth, and the higher the stratum of air the more decidedly positive is its electrical condition. The source of atmospheric electricity is traced by Lavoisier, Laplace, Volta, and De Saussure to evaporation from the surface of the earth, the effect of which is to convey one kind of electricity upward with the vapor, leaving the other with the fluid.

But, as shown by Pouillet in 1823, this effect does not take place unless the evaporation is accompanied with chemical decomposition, as when it occurs from saline mixtures, from the surface of heated iron, which becomes oxidized, and more especially when the vapor proceeds from the leaves of growing plants. Combustion also is a source of atmospheric electricity, as is seen upon a large scale in the constant flashes of lightning that sometimes play around the summits of volcanoes during their eruptions. The rushing of currents of wind past each other, or against opposing objects, also generates electricity by the friction it occasions. The descent of the rain drops develops negative electricity in the air, and the same effect is observed in the vicinity of waterfalls, the air for several hundred feet distant being filled with negative electricity. To this cause is probably to be attributed the highly excited condition of the atmosphere during thunder storms, and the frequent alternations then observed of positive and negative indications.

However the elec-trical condition of the clouds is produced, the surface beneath assumes the opposite electrical state, the stratum of air between acting like the insulating glass plate between the two metallic surfaces; and when at last the attraction between the two opposite electricities becomes too strong for the interposed medium to resist, they rush together, producing the disruptive discharge accompanied with the flash and report. With a good conductor passing from the cloud to the earth, the electrical equilibrium would be silently restored, as a Leyden jar is quietly discharged by connecting its inner and outer surfaces with a wire pointed at each end. But if an imperfect conductor is interposed, the electricity, seeking to follow this, may produce the most violent effects, and these are exhibited at the points where the continuity of the conductor is imperfect or interrupted. This is well illustrated in the common experiment with the model of a house loosely put together and furnished with an interrupted rod, through which an electrical shock is conveyed. The effect is to throw the model into pieces; but when the same experiment is tried upon a complete rod, the discharge takes place without violent action.

Sir W. Snow Harris also illustrates the effect of an interrupted conductor by scattering bits of gold leaf upon paper, and passing along them an electrical discharge, sufficient to burn the gold and blacken the paper. But it is observed in this experiment that only those bits are burned, and the portions of them only, which lie along the line of most perfect conduction or of least resistance; the paper too will be nowhere blackened except on this line. Similar phenomena are observed upon a large scale in almost every instance of a house being struck by lightning. The path of the electrical current is traced along the best conductors, and as the lightning passes from one to another the most destructive effects are observed in these breaks. Imperfect conductors lying near are shattered to pieces or scattered about, and the effects of intense heat are developed where the current is most obstructed. The animal system offering a good conductor, the lightning leaves more imperfect ones to pass by this on its course, and thus men and beasts are frequently struck when standing near projecting objects, as trees, that present themselves as convenient mediums for the reestablishment of the electrical equilibrium. - Franklin, having satisfied himself of the identity of lightning and electricity, was not long in drawing from his discovery practical results of immense importance in protecting buildings from the stroke of lightning; and he thus announced in his "Poor Richard's Almanac" for 1753 his invention of the lightning rod, the description being nearly as complete and exact in all its essential particulars as could now be given after the experience and trials of more than a century: "How to Secure Houses, etc, from Lightning. It has pleased God, in his goodness to mankind, at length to discover to them the means of securing their habitations and other buildings from mischief by thunder and lightning.

The method is this: Provide a small iron rod (it may be made of the rod iron used by the nailers), but of such a length that one end being 3 or 4 ft. in the moist ground, the other may be 6 or 8 ft. above the highest part of the building. To the upper end of the rod fasten about a foot of brass wire, the size of a common knitting needle, sharpened to a fine point; the rod may be secured to the house by a few small staples. If the house or barn be long, there may be a rod and point at each end and a middling wire along the ridge from one to the other. A house thus furnished will not be damaged by the lightning, it being attracted by the points, and passing through the metal into the ground without hurting anything. Vessels also having a sharp-pointed rod fixed on the top of their masts, with a wire from the foot of the rod reaching down round one of the shrouds to the water, will not be hurt by lightning." Thus Franklin merited the words of the French medal subsequently struck in his honor, Eripuit coelo fulmen, though from a passage found among the fragments of Ctesias (Photii Bibliotheca), it would seem that some knowledge was possessed by the ancients, 400 years before the Christian era, of the effect of iron rods in averting the lightning.

The writer in this passage makes mention of a fountain in India, from the bottom of which was obtained a kind of iron, which being set in the ground averted clouds, hail, and lightning. Various modifications in the construction of the rod have since been proposed, and copper has been advantageously substituted for iron, as in those planned by Sir W. Snow Harris for the use of the ships of the royal navy. These protectors are in bands of copper, overlapping each other so as to break joints, and are let in to the after side of each mast. They pass down to the keel, and are continued through this by copper bolts into the water; they also connect with copper bands laid under the deck beams and continued through the side of the ship. Harris also made conductors for buildings of copper pipes firmly screwed together, and furnished at top with a pointed extremity 1 1/2 ft. long and £ in. in diameter. The tubes for a given amount of metal expose the greatest surface, and thus furnish the maximum capacity of conduction of the electrical current.

Copper moreover conveys the current more freely than iron in the proportion of 12 to 2 1/2. This is an important feature, inasmuch as, having no measure of the power of the current that may strike the rod, we should provide one of sufficient size for any stroke. An iron wire may be entirely inefficient, and melt beneath the electrical current, or this may be divided and bound off to other more or less perfect conductors near the rod. It is this inefficiency or imperfect construction of rods in use that has led many to question the value of any metallic conductors, and even to imagine that they all serve to attract lightning, and thus increase the danger. Their office is that of conductors of the electrical current, as the bed of a river presents itself for the flow of the aqueous current. Each may act as a safety valve to its respective current when this is impelled with unusual violence; and in case of obstruction to either disastrous consequences may ensue. Iron rods loosely jointed together, and perhaps rusty in the joints, furnish a bad conveyance for the electrical current; and if not continued down into moist ground, and there branching out, the passage of the electricity into the earth may not be so free as by other conductors in the building itself.

Wrought-iron rods are commonly used in the United States on account of their greater cheapness. They should be at least three fourths of an inch in diameter, and in as long pieces as is practicable. The joints that cannot be avoided should be very securely fitted, so that the two ends are brought into close contact, and touch each other for several inches in length. The branching terminations in the ground may very well be filled around with charcoal, which is a good conductor, and also protects the rod to some extent from rusting. The points at the top may be protected from rust by gold leaf, and the whole rod may be painted with black paint having lampblack for its chief ingredient. A good rod may be secured without danger to the building by wooden clamps with iron fastenings, or even with iron staples. Glass insulators are useless, for when wet they become conductors. It is recommended by some persons, that as the greatest number of thunder storms in this country come from the northwest, the conductors should be placed on the side of the building exposed to their first approach.

But it is particularly important that every prominent elevated point of a large building should be protected by its own rod, and it is well to connect all the rods together, and to have two or more stems running into the ground. It is very uncertain how large an area a rod of given height can protect. Different French electricians have variously rated it as a circular space of radius from one to three times the height of the rod above the highest point to which it is attached; but little confidence can be placed in these conclusions. The opposite electricities, the concurrence of which produces the discharge, are far from being uniformly distributed through the atmosphere, and their point of rushing together may not be in any way under the influence of a rod directed into the air in its vicinity. The position of the excited masses may be favorable for a lateral discharge, and such have been known to pass horizontally through the atmosphere long distances, and to strike with destructive violence objects lying in their path. And as evidence of the protecting influence of a single point not reaching to any considerable distance, a case is cited of the foremast of a ship being struck, causing serious damage to the vessel, when the mainmast was provided with a conductor.

Hence the importance of points upon the rods along the salient parts of buildings they are designed to protect. By the great multiplication of conductors the accumulation of opposite electricities in quantities sufficient to produce destructive discharges is prevented; and thus it is that houses in cities are rarely struck, or vessels where many are lying together in the docks. Isolated houses are more commonly the objects of the lightning stroke; and it is observed that particular localities are subject to be repeatedly struck at different periods; other spots are singularly free from such visitations. Chimneys from which hot and rarefied air is ascending into the atmosphere, and barns stored with new hay, the vapors from which also produce warm ascending currents, are especially liable to be struck. It is prudent for persons in a building to avoid being near a chimney or the walls, or in close proximity to metallic bodies, along which the lightning may find the readiest path. The greatest safety would be found, as stated by Franklin, in lying in a hammock suspended by silken cords in the middle of a large apartment.

Insulation by placing one's self upon a feather bed, or any poor conductor, is also a protection, not however complete unless the head is covered by some non-conducting substance. The efficacy of lightning rods is sometimes doubted, and an idea entertained that the rod often proves dangerous by attracting the stroke. It is difficult to say how many buildings have been saved by rods, as it is impossible to say what might or might not have taken place in their absence. It appears however that in Germany statistics have lately been furnished by insurance companies which support the opinion that rods offer a great degree of protection. It has been found that the first point struck by lightning is that at which the greatest heating effect is produced, and that if no inflammable materials are present there, the danger of fire following the stroke is greatly diminished. Inflammable vapors form better conductors than the air, and if no rods are furnished to buildings when such vapors are issuing they are liable to be struck, and when struck more liable to take fire than if supplied with rods. The English association of telegraph engineers have furnished still more valuable information.

The poles of their lines were frequently struck until they mounted them with wire running from the top to the ground. They have found that it is well to have a large mass of metal in the ground connected with the wire, and that the latter should be as straight as possible. The rod should be continuous, and present no points except at the top; insulation under such circumstances is not necessary. - One of the most useful works for reference in regard to lightning and lightning rods is the treatise of Sir W. Snow Harris "On the Nature of Thunder Storms, and on the Means of Protecting Buildings and Shipping against the Destructive Effects of Lightning" (London, 1843).