Read Ebook: Lightning Thunder and Lightning Conductors by Molloy Gerald

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A conjecture has recently been started that electricity may be generated by the mere impact of minute particles of water vapor against minute particles of air. If this conjecture could be established as a fact, it would be amply sufficient to account for all the electricity of the atmosphere. From the very nature of a gas, the molecules of which it is composed are forever flying about with incredible velocity; and therefore the particles of water vapor and the particles of air, which exist together in the atmosphere, must be incessantly coming into collision. Hence, however small may be the charge of electricity developed at each individual impact, the total amount generated over any considerable area, in a single day, must be very great indeed. It is evident, however, that this method of explaining the origin of atmospheric electricity can only be regarded as, at best, a probable hypothesis, until the assumption on which it rests is supported by the evidence of observation or experiment.

This explanation, which traces the exceedingly high potential of lightning to the building up of rain drops in the thundercloud, suggests a reason why it so often happens that immediately after a flash of lightning "the big rain comes dancing to the earth." The potential has been steadily rising as the drops have been getting larger and larger, until at length the potential has become so high that the thundercloud is able to discharge itself, and almost at the same moment the drops have become so large that they can no longer be held aloft against the attracting force of gravity.

To understand why the sound of thunder reaches the ear as a prolonged peal, we have only to remember that sound takes time to travel. Since a flash of lightning is practically instantaneous, we may assume that the sound is produced at the same moment all along the line of discharge. But the sound waves, setting out at the same moment from all points along the line of discharge, must reach the ear in successive instants of time, arriving first from that point which is nearest to the observer, and last from that point which is most distant. Suppose, for example, that the nearest point of the flash is a mile distant from the observer, and the farthest point two miles--the sound will take about five seconds to come from the nearest point, and about ten seconds to come from the farthest point; and moreover, in each successive instant from the time the first sound reaches the ear, sound will continue to arrive from the successive points between. Therefore the thunder, though instantaneous in its origin, will reach the ear as a prolonged peal extending over a period of five seconds.

The next cause that I would mention is the zigzag path of the lightning discharge. To make clear to you the influence of this circumstance, I must ask your attention for a moment to the diagram on next page. Let the broken line represent the path of a flash of lightning, and let O represent the position of an observer. The sound will reach him first from the point A, which is nearest to him, and then it will continue to arrive in successive instants from the successive points along the line A N and along the line A M, thus producing the effect of a continuous peal. Meanwhile the sound waves have been traveling from the point B, and in due time will reach the observer at O. Coming as they do in a different direction from the former, they will strike the ear as the beginning of a new peal which, in its turn, will be prolonged by the sound waves arriving, in successive instants, from the successive points along the line B M and B H. A little later, the sound will arrive from the more distant point C, and a third peal will begin. And so there will be several distinct peals proceeding, so to speak, from several distinct points in the path of the lightning flash.

A third cause to which the succession of peals may be referred is to be found in the minor electrical discharges that must often take place within the thundercloud itself. A thundercloud is not a continuous mass like the metal cylinder of this electric machine--it has many outlying fragments, more or less imperfectly connected with the principal body. Moreover, the material of which the cloud is composed is only a very imperfect conductor as compared with our brass cylinder. For these two reasons it must often happen, about the time a flash of lightning passes, that different parts of the cloud will be in such different electrical conditions as to give rise to electrical discharges within the cloud itself. Each of these discharges produces its own peal of thunder; and thus we may have a number of minor peals, sometimes preceding and sometimes following the great crash which is due to the principal discharge.

Lastly, the influence of echo has often a considerable share in multiplying the number of peals of thunder. The waves of sound, going forth in all directions, are reflected from the surfaces of mountains, forests, clouds, and buildings, and coming back from different quarters, and with varying intensity, reach the ear like the roar of distant artillery. The striking effect of these reverberations in a mountain district has been described by a great poet in words which, I daresay, are familiar to most of you:

"Far along, From peak to peak, the rattling crags among, Leaps the live thunder! Not from one lone cloud, But every mountain now has found a tongue, And Jura answers from her misty shroud Back to the joyous Alps, that call to her aloud!"

But there is another influence at work which must produce variations in the loudness of a peal of thunder, even though the sound waves, set in motion by the lightning, were everywhere of equal intensity. This influence depends on the position of the observer in relation to the path of the lightning flash. At one part of its course the lightning may follow a path which remains for a certain length at nearly the same distance from the observer; then all the sound produced along this length will reach the observer nearly at the same moment, and will burst upon the ear with great intensity. At another part, the lightning may for an equal length go right away from the observer; and it is evident that the sound produced along this length will reach the observer in successive instants, and consequently produce an effect comparatively feeble.

With a view to investigate this interesting question a little more closely, let me suppose the position of the observer taken as a centre, and a number of concentric circles drawn, cutting the path of the lightning flash, and separated from one another by a distance of 110 feet, measured along the direction of the radius. It is evident that all the sound produced between any two consecutive circles will reach the ear within a period which must be measured by the time that sound takes to travel 110 feet, that is, within the tenth of a second. Hence, in order to determine the quantity of sound that reaches the ear in successive periods of one-tenth of a second, we have only to observe how much is produced between each two consecutive circles. But on the supposition that the sound waves, set in motion by the flash of lightning, are of equal intensity at every point of its path, it is clear that the quantity of sound developed between each two consecutive circles will be simply proportional to the length of the path enclosed between them.

With these principles established, let us now follow the course of a peal of thunder, in the diagram before us. This broken line, drawn almost at random, represents the path of a flash of lightning; the observer is supposed to be placed at O, which is the centre of the concentric circles; these circles are separated from one another by a distance of 110 feet, measured in the direction of the radius; and we want to consider how any one peal of thunder may vary in loudness in the successive periods of one-tenth of a second.

Let us take, for example, the peal which begins when the sound waves reach the ear from the point A. In the first unit of time the sound that reaches the ear is the sound produced along the lines A B and A C; in the second unit, the sound produced along the lines B D and C E; in the third unit, the sound produced along D F and E G. So far the peal has been fairly uniform in its intensity; though there has been a slight falling off in the second and third units of time, as compared with the first. But in the fourth unit there is a considerable falling away of the sound; for the line F K is only about one-third as long as D F and E G taken together; therefore the quantity of sound that reaches the ear in the fourth unit of time is only one-third of that which reaches it in each of the three preceding units; and consequently the sound is only one-third as loud. In the fifth unit, however, the peal must rise to a sudden crash; for the portion of the lightning path inclosed between the fifth and sixth circles is about six times as great as that between the fourth and fifth; therefore the intensity of the sound will be suddenly increased about six-fold. After this sudden crash, the sound as suddenly dies away in the sixth unit of time; it continues feeble as the path of the lightning goes nearly straight away from the observer; it swells again slightly in the ninth unit of time; and then continues without much variation to the end. This is only a single illustration, but it seems quite sufficient to show that the changes of intensity in a peal of thunder must be largely due to the position of the spectator in relation to the several parts of the lightning flash.

You will observe also that, by repeating this observation, we can determine whether the thundercloud is coming toward us, or going away from us. So long as the interval between each successive flash and the corresponding peal of thunder, continues to get shorter and shorter, the thundercloud is approaching; when the interval begins to increase, the thundercloud is receding from us, and the danger is passed.

The crash of thunder is terrific when the lightning is close at hand; but it is a curious fact, that the sound does not seem to travel as far as the report of an ordinary cannon. We have no authentic record of thunder having been heard at a greater distance than from twelve to fifteen miles, whereas the report of a single cannon has been heard at five times that distance; and the roar of artillery, in battle, at a greater distance still. On the occasion of the Queen's visit to Cherbourg, in August, 1858, the salute fired in honor of her arrival was heard at Bonchurch, in the Isle of Wight, a distance of sixty miles. It was also heard at Lyme Regis, in Dorsetshire, which is eighty-five miles from Cherbourg, as the crow flies; and we are told that, not only was it audible in its general effect, but the report of individual guns was distinctly recognized. The artillery of Waterloo is said to have been heard at the town of Creil, in France, 115 miles from the field of battle; and the cannonading at the siege of Valenciennes, in 1793, was heard, from day to day, at Deal, on the coast of England, a distance of 120 miles.

So far, I have endeavored to set forth some general ideas on the nature and origin of lightning, and of the thunder that accompanies it. In my next Lecture I propose to give a short account of the destructive effects of lightning, and to consider how these effects may best be averted by means of lightning conductors.

NOTE TO PAGE 20.

ON THE HIGH POTENTIAL OF A FLASH OF LIGHTNING.

FOOTNOTES:

"Il y a des grands seigneurs dont il ne faut approcher qu'avec d'extr?mes pr?cautions. Le tonnerre est de ce nombre."--Dict. Philos. art. Foudre.

Electricit? Statique, ii., 561.

Deschanel's Natural Philosophy, Sixth Edition, p. 641.

Fragments of Science, Fifth Edition, p. 311.

Lecture on Thunderstorms, Nature, vol. xxii., p. 341.

Third Series, vol. v., p. 161.

Phil. Trans. Royal Society, 1834, vol. cxxv., pp. 583-591.

Nature, vol. xxviii., p. 54.

See, however, an attempt to account for this phenomenon in De Larive's Treatise on Electricity, London, 1853-8, vol. iii., pp. 199, 200; and another, quite recently, by Mr. Spottiswoode, in a Lecture on the Electrical Discharge, delivered before the British Association at York, in September, 1881, and published by Longmans, London, p. 42. See also, for recent evidence regarding the phenomenon itself, Scott's Elementary Meteorology, pp. 175-8.

See Jamin, "Cours de Physique," i., 480-1; Tomlinson, "The Thunderstorm," Third Edition, pp. 95-103; "Thunderstorms," a Lecture by Professor Tait, Nature, vol. xxii., p. 356.

Professor Tait, On Thunderstorms, Nature, vol. xxii., pp. 436-7.

See note at the end of this Lecture, p. 26.

See Tomlinson, The Thunderstorm, pp. 87-9.

See Tait on Thunderstorms, Nature, vol. xxii., p. 436.

LIGHTNING CONDUCTORS.

The effects of lightning, on the bodies that it strikes, are analogous to those which may be produced by the discharge of our electric machines and Leyden jar batteries. When the discharge of a battery traverses a metal conductor of sufficient dimensions to allow it an easy passage, it makes its way along silently and harmlessly. But if the conductor be so thin as to offer considerable resistance, then the conductor itself is raised to intense heat, and may be melted, or even converted into vapor, by the discharge.

On opposite page is shown a board on which a number of very thin wires have been stretched, over white paper, between brass balls. The wires are so thin that the full charge of the battery before you, which consists of nine large Leyden jars, is quite sufficient to convert them in an instant into vapor. I have already, on former occasions, sent the charge through two of these wires, and nothing remains of them now but the traces of their vapor, which mark the path of the electric discharge from ball to ball. At the present moment the battery stands ready charged, and I am going to discharge it through a third wire, by means of this insulated rod which I hold in my hand. The discharge has passed; you saw a flash, and a little smoke; and now, if you look at the paper, you will find that the wire is gone, but that it has left behind the track of its incandescent vapor, marking the path of the discharge.

Again, the electric discharge, passing through a bad conductor, produces mechanical disturbance, and, if the substance be combustible, often sets it on fire. So, too, as you know, the lightning flash, falling on a church spire, dashes it to pieces, knocking the stones about in all directions, while it sets fire to ships and wooden buildings; and more than once it has caused great devastation by exploding powder magazines.

Let me give you one or two examples: In January, 1762, the lightning fell on a church tower in Cornwall, and a stone--three hundred-weight--was torn from its place and hurled to a distance of 180 feet, while a smaller stone was projected as far as 1,200 feet from the building. Again, in 1809, the lightning struck a house not far from Manchester, and literally moved a massive wall twelve feet high and three thick to a distance of several feet. You may form some conception of the enormous force here brought into action, when I tell you that the total weight of mason-work moved on this occasion was not less than twenty-three tons.

The church of St. George, at Leicester, was severely damaged by lightning on the 1st of August, 1846. About 8 o'clock in the evening the rector of the parish saw a vivid streak of light darting with incredible velocity against the upper part of the spire. "For the distance of forty feet on the eastern side, and nearly seventy on the west, the massive stonework of the spire was instantly rent asunder and laid in ruins. Large blocks of stone were hurled in all directions, broken into small fragments, and in some cases, there is reason to believe, reduced to powder. One fragment of considerable size was hurled against the window of a house three hundred feet distant, shattering to pieces the woodwork, and strewing the room within with fine dust and fragments of glass. It has been computed that a hundred tons of stone were, on this occasion, blown to a distance of thirty feet in three seconds. In addition to the shivering of the spire, the pinnacles at the angles of the tower were all more or less damaged, the flying buttresses cracked through and violently shaken, many of the open battlements at the base of the spire knocked away, the roof of the church completely riddled, the roofs of the side entrances destroyed, and the stone staircases of the gallery shattered."

Lightning has been at all times the cause of great damage to property by its power of setting fire to whatever is combustible. Fuller says, in his Church History, that "scarcely a great abbey exists in England which once, at least, has not been burned by lightning from heaven." He mentions, as examples, the Abbey of Croyland twice burned, the Monastery of Canterbury twice, the Abbey of Peterborough twice; also the Abbey of St. Mary's, in Yorkshire, the Abbey of Norwich, and several others. Sir William Snow Harris, writing about twenty years ago, tells us that "the number of churches and church spires wholly or partially destroyed by lightning is beyond all belief, and would be too tedious a detail to enter upon. Within a comparatively few years, in 1822 for instance, we find the magnificent Cathedral of Rouen burned, and, so lately as 1850, the beautiful Cathedral of Saragossa, in Spain, struck by lightning during divine service and set on fire. In March of last year a dispatch from our Minister at Brussels, Lord Howard de Walden, dated the 24th of February, was forwarded by Lord Russell to the Royal Society, stating that, on the preceding Sunday, a violent thunderstorm had spread over Belgium; that twelve churches had been struck by lightning; and that three of these fine old buildings had been totally destroyed."

Even in our own day the destruction caused by fires produced through the agency of lightning is very great--far greater than is commonly supposed. No general record of such fires is kept, and consequently our information on the subject is very incomplete and inexact. I may tell you, however, one small fact which, so far as it goes, is precise enough and very significant. In the little province of Schleswig-Holstein, which occupies an area less than one-fourth of the area of Ireland, the Provincial Fire Assurance Association has paid in sixteen years, for damage caused by lightning, somewhat over ?100,000, or at the rate of more than ?6,000 a year. The total loss of property every year in this province, due to fires caused by lightning, is estimated at not less than ?12,500.

This will probably appear to you a very small affair, when compared with the tearing asunder of solid masonry, and the hurling about of stones by the ton weight. No doubt it is; and that is just one of the lessons we have to learn from the experiment we have made. For, not only does it show us that effects of this kind may be caused by electricity artificially produced, but it brings home forcibly to the mind how incomparably more powerful is the lightning of the clouds than the electricity of our machines.

The property which electricity has of setting fire to combustible substances may be easily illustrated. This india-rubber tube is connected with the gas pipe under the floor, and to the end of the tube is fitted a brass stop-cock which I hold in my hand. I open the cock, and allow the jet of gas to flow toward the conductor of Carr?'s machine, while my assistant turns the handle; a spark passes, and the gas is lit. Again, my assistant stands on this insulating stool, placing his hand on the large conductor of the machine, while I turn the handle. His body becomes electrified, and when he presents his knuckle to this vessel of spirits of wine, which is electrically connected with the earth, a spark leaps across, and the spirits of wine are at once in a blaze. Once more; I tie a little gun-cotton around one knob of the discharging rod, and then use it to discharge a small Leyden jar; at the moment of the discharge the gun-cotton is set on fire.

So far as can be gathered from the existing sources of information, it would seem that the number of persons killed by lightning is, on the whole, about one in three of those who are struck. The rest are sometimes only stunned, sometimes more or less burned, sometimes made deaf for a time, sometimes partially paralyzed. On particular occasions, however, especially when the lightning falls on a large assembly of people, the number of persons struck down and slightly injured, in proportion to the number killed, is very much increased.

An interesting case of this kind is reported by Mr. Tomlinson. "On the twenty-ninth of August, 1847, at the parish church of Welton, Lincolnshire, while the congregation were engaged in singing the hymn before the sermon, and the Rev. Mr. Williamson had just ascended the pulpit, the lightning was seen to enter the church from the belfry, and instantly an explosion occurred in the centre of the edifice. All that could move made for the door, and Mr. Williamson descended from the pulpit, endeavoring to allay the fears of the people. But attention was now called to the fact that several of the congregation were lying in different parts of the church, apparently dead, some of whom had their clothing on fire. Five women were found injured, and having their faces blackened and burned, and a boy had his clothes almost entirely consumed. A respected old parishioner, Mr. Brownlow, aged sixty-eight, was discovered lying at the bottom of his pew, immediately beneath one of the chandeliers, quite dead. There were no marks on the body, but the buttons of his waistcoat were melted, the right leg of his trousers torn down, and his coat literally burnt off. His wife in the same pew received no injury."

Many experiments have been devised to illustrate this theory of Lord Mahon. But the best illustration I know is furnished by this electric machine of Carr?'s. If you stand near one end of the large conductor when the machine is in action and sparks are taken from the other end, you will feel a distinct electric shock every time a spark passes. The large conductor here takes the place of the cloud, the spark that passes at one end represents the flash of lightning, and the observer at the other end gets the return shock, though he is at a considerable distance from the point where the flash is seen.

An experiment of this kind, of course, cannot be made sensible to a large audience like the present. But I can give you a good idea of the effect by means of this tuft of colored papers. While the machine is in action I hold the tuft of papers near that end of the conductor which is farthest from the point where the discharge takes place. You see the paper ribbons are electrified by induction, and, in virtue of mutual repulsion, stand out from one another "like quills upon the fretful porcupine." But, when a spark passes, the inductive action ceases, the paper ribbons cease to be electrified, and the whole tuft suddenly collapses into its normal state.

While fully accepting Lord Mahon's theory of the return shock as perfectly good so far as it goes, I would venture to point out another influence which must often contribute largely to produce the effect in question, and which is not dependent on the form of the cloud. It may easily happen, from the nature of the surface in the district affected by a thundercloud, that the point of most intense electrification--say E in the figure--is in good electrical communication with a distant point, such as F, while it is very imperfectly connected with a much nearer point, D. In such a case it is evident that bodies at F will share largely in the highly-electrified condition of E, and also share largely in the sudden change of that condition the moment the flash of lightning passes; whereas bodies at D will be less highly electrified before the discharge, and less violently disturbed when the discharge takes place.

This principle may be illustrated by a very simple experiment. Here is a brass chain about twenty feet long. One end of it I hand to any one among the audience who will kindly take hold of it; the other end I hold in my hand. I now stand near the conductor of the machine; and will ask some one to stand about ten feet away from me, near the middle of the chain, but without touching it. Now observe what happens when the machine is worked and I take a spark from the conductor: My friend at the far end of the chain, twenty feet away, gets a shock nearly as severe as the one I get myself, because he is in good electrical communication with the point where the discharge takes place. But my more fortunate friend, who is ten feet nearer to the flash, is hardly sensible of any effect, because he is connected with me only through the floor of the hall, which is, comparatively speaking, a bad conductor of electricity.

Franklin's lightning rods were soon adopted in America; and he himself contributed very much to their popularity by the simple and lucid instructions he issued every year, for the benefit of his countrymen, in the annual publication known as "Poor Richard's Almanac." It is very interesting at this distance of time to read the homely practical rules laid down by this great philosopher and statesman; and, though some modifications have been suggested by the experience of a hundred and thirty years, especially as regards the dimensions of the lightning conductor, it is surprising to find how accurately the general principles of its construction, and of its action, are here set forth.

"It has pleased God," he says, "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, which may be made of the rod-iron used by nailors, but of such a length that one end being three or four feet in the moist ground, the other may be six or eight feet 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 on 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 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."

The first public building protected by a lightning rod in England was St. Paul's Cathedral, in London. On the eighteenth of June, 1764, the beautiful steeple of Saint Bride's Church, in the city, was struck by lightning and reduced to ruin. This incident awakened the attention of the dean and chapter of St. Paul's to the danger of a similar calamity, which seemed, as it were, impending over their own church. After long deliberation, they referred the matter to the Royal Society, asking for advice and instruction. A committee of scientific men was appointed by the Royal Society to consider the question. Benjamin Franklin himself, who happened to be in London at the time, as the representative of the American States in their dispute with England, was nominated a member of the committee. And the result of its deliberation was that, in the year 1769, a number of lightning conductors were erected on St. Paul's Cathedral.

It was on this occasion that arose the celebrated controversy about the respective merits of points and balls. Franklin had recommended a pointed conductor; but some members of the committee were of opinion that the conductor should end in a ball and not in a point. The decision of the committee was in favor of Franklin's opinion, and pointed conductors were accordingly adopted for St. Paul's Cathedral. But the controversy did not end here. The time was one of great political excitement, and party spirit infused itself even into the peaceful discussions of science. The weight of scientific opinion was on the side of Franklin; but it was hinted, on the other side, that the pointed conductors were tainted with republicanism, and pregnant with danger to the empire. As a rule, the whigs were strongly in favor of points; while the Tories were enthusiastic in their support of balls.

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