Chapter II: Part 2

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.

=Distance of a Flash of Lightning.=--I need hardly remind you that, by observing the interval that elapses between the flash of lightning and the peal of thunder that follows it, we may estimate approximately the distance of the nearest point of the discharge. Light travels with such amazing velocity that we may assume, without any sensible error, that we see the flash of lightning at the very moment in which the discharge takes place. But sound, as we have seen, takes a sensible time to travel even short distances; and therefore a measurable interval almost always elapses between the moment in which the flash is seen and the moment in which the peal of thunder first reaches the ear. And the distance through which sound travels in this interval will be the distance of the nearest point through which the discharge has passed. Now, the velocity of sound in air varies slightly with the temperature; but, at the ordinary temperature of our climate, we shall not be far astray if we allow 1,100 feet for every second, or about one mile for every five seconds.

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.[15]

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.

The potential of an electrified sphere is equal to the quantity of electricity with which the sphere is charged, divided by the radius of the sphere. Now the minute cloud particles, which go to make up a drop of rain, may be taken to be very small spheres; and if _v_ represent the potential of each one, _q_ the quantity of electricity with which it is charged, and _r_ the radius of the sphere, we have _v_ = _q_/_r_. Suppose 1,000 of these cloud particles to unite into one; the quantity of electricity in the drop, thus formed, will be 1,000_q_; and the radius, which increases in the ratio of the cube root of the volume, will be 10_r_. Therefore the potential of the new sphere will be 1000_q_/10_r_, or 100_q/r_; that is to say, it will be 100 times as great as the potential of each of the cloud particles which compose it. When a million of cloud particles are blended into a single drop, the same process will show that the potential has been increased ten thousandfold; and when a drop is produced by the agglomeration of a million of millions of cloud particles, the potential of the drop will be a hundred million times as great as that of the individual particles.[16]

FOOTNOTES:

[1] “Il y a des grands seigneurs dont il ne faut approcher qu’avec d’extrêmes précautions. Le tonnerre est de ce nombre.”--Dict. Philos. art. Foudre.

[2] Electricité Statique, ii., 561.

[3] Deschanel’s Natural Philosophy, Sixth Edition, p. 641.

[4] Fragments of Science, Fifth Edition, p. 311.

[5] Lecture on Thunderstorms, Nature, vol. xxii., p. 341.

[6] Third Series, vol. v., p. 161.

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

[8] In experiments with a Leyden jar, Feddersen has shown that the duration of the discharge is increased, not only by increasing the striking distance, but also by increasing the size of the jar. Now, a flash of lightning may be regarded as the discharge of a Leyden jar of immense size, with an enormous striking distance; and therefore we should expect that the duration of the discharge should be greatly prolonged. See _American Journal of Science and Arts_, Third Series, vol. i., p. 15.

[9] See original paper by Swan, Trans. Royal Society, Edinburgh, 1849, vol. xvi., pp. 581-603; also, a second paper, _ib._ 1861, vol. xxii., pp. 33-39.

[10] Nature, vol. xxviii., p. 54.

[11] 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.

[12] 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.

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

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

[15] See Tomlinson, The Thunderstorm, pp. 87-9.

[16] See Tait on Thunderstorms, Nature, vol. xxii., p. 436.

LECTURE II.

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.

=Destruction of Buildings by Lightning.=--We learn from this experiment that the electricity stored up in our battery passes, without visible effect, through the stout wire of a discharging rod, but that it instantly converts into vapor the thin wire stretched across the spark board. And so it is with a flash of lightning. It passes harmlessly, as every one knows, through a stout metal rod, but when it comes across bell wires or telegraph wires, it melts them, or converts them into vapor. On the sixteenth of July, 1759, a flash of lightning struck a house in Southwark, on the south side of London, and followed the line of the bell wire. After the lightning had passed, the wire was no longer to be found; but the path of the lightning was clearly marked by patches of vapor which were left, here and there, adhering to the surface of the wall. In the year 1754, the lightning fell on a bell tower at Newbury, in the United States of America, and having dashed the roof to pieces, and scattered the fragments about, it reached the bell. From this point it followed an iron wire, about as thick as a knitting needle, melting it as it passed along, leaving behind a black streak of vapor on the surface of the walls.

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.”[17]

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.”[18]

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.[19]

=Destruction of Ships at Sea.=--The destructive effects of lightning on ships at sea, before the general adoption of lightning conductors, seems almost incredible at the present day. From official records it appears that the damage done to the Royal Navy of England alone involved an expenditure of from £6,000 to £10,000 a year. We are told by Sir William Snow Harris, who devoted himself for many years to this subject with extraordinary zeal and complete success, that between the year 1810 and the year 1815--that is, within a period of five years--“no less than forty sail of the line, twenty frigates, and twelve sloops and corvettes were placed _hors de combat_ by lightning. In the merchant navy, within a comparatively small number of years, no less than thirty-four ships, most of them large vessels with rich cargoes, have been totally destroyed--been either burned or sunk--to say nothing of a host of vessels partially destroyed or severely damaged.”[20]

And these statements, be it observed, take no account of ships that were simply reported as missing, some of which, we can hardly doubt, were struck by lightning in the open sea, and went down with all hands on board. A famous ship of forty-four guns, the _Resistance_, was struck by lightning in the Straits of Malacca, and the powder magazine exploding, she went to the bottom. Of her whole crew only three were saved, who happened to be picked up by a passing boat. It has been well observed that, were it not for these three chance survivors, nothing would have been known concerning the fate of the vessel, and she would have been simply recorded as missing in the Admiralty lists.

Nothing is more fearful to contemplate than the scene on board a ship when she is struck by lightning in the open sea, with the winds howling around, the waves rolling mountains high, the rain coming down in torrents, and the vivid flashes lighting up the gloom at intervals, and carrying death and destruction in their track. I will read you one or two brief accounts of such a scene, given in the pithy but expressive language of the sailor. In January, 1786, the _Thisbe_, of thirty-six guns, was struck by lightning off the coast of Scilly, and reduced to the condition of a wreck. Here is an extract from the ship’s log: “Four A. M., strong gales; handed mainsail and main top-sail; hove to with storm staysails; blowing very heavy, S. E. 4.15, a flash of lightning, with tremendous thunder, disabled some of our people. A second flash set the mainsail, main-top, and mizen staysails on fire. Obliged to cut away the mainmast; this carried away mizen top-mast and fore top-sail yard. Found foremast also shivered by the lightning. Fore top-mast went over the side about 9 A. M. Set the foresail.”[21]

A few years later, in March, 1796, the _Lowestoffe_ was struck in the Mediterranean, and we read as follows in the log of the ship: “North end of Minorca; heavy squalls; hail, rain, thunder, and lightning. 12.15, ship struck by lightning, which knocked three men from the masthead, one killed. 12.30, ship again struck; main top-mast shivered in pieces; many men struck senseless on the decks. Ship again struck, and set on fire in the masts and rigging; mainmast shivered in pieces; fore top-mast shivered; men benumbed on the decks, and knocked out of the top; one man killed on the spot. 1.30, cut away the mainmast; employed clearing wreck. 4, moderate; set the foresail.”[22]

Again, in 1810, the _Repulse_, a ship of seventy-four guns, was struck, off the coast of Spain. “The wind had been variable in the morning--and at 12.35 there was a heavy squall, with rain, thunder, and lightning. The ship was struck by two vivid flashes of lightning, which shivered the maintop-gallant mast, and severely damaged the mainmast. Seven men were killed on the spot; three others only survived a few days; and ten others were maimed for life. After the second discharge the rain fell in torrents. The ship was more completely crippled than if she had been in action, and the squadron, then engaged on a critical service, lost for a time one of its fastest and best ships.”[23]

=Destruction of Powder Magazines.=--Not less appalling is the devastation caused by lightning when it falls on a powder magazine. Here is a striking example: On the eighteenth of August, 1769, the tower of St. Nazaire, at Brescia, was struck by lightning. Underneath the tower about 200,000 pounds of gunpowder, belonging to the Republic of Venice, were stored in vaults. The powder exploded, leveling to the ground a great part of the beautiful city of Brescia, and burying thousands of its inhabitants in the ruins. It is said that the tower itself was blown up bodily to a great height in the air, and came down in a shower of stones. This is, perhaps, the most fearful disaster of the kind on record. But we are not without examples in our own times. In the year 1856 the lightning fell on the Church of St. John, in the Island of Rhodes. A large quantity of gunpowder had been deposited in the vaults of the church. This was ignited by the flash; the building was reduced to a mass of ruins, a large portion of the town was destroyed, and a considerable number of the inhabitants were killed. Again, in the following year, the magazine of Joudpore, in the Bombay Presidency, was struck by lightning. Many thousand pounds of gunpowder were blown up, five hundred houses were destroyed, and nearly a thousand people are said to have been killed.[24]

=Experimental Illustrations.=--And now, before proceeding further, I will make one or two experiments, with a view of showing that the electricity of our machines is capable of producing effects similar to those produced by lightning, though immeasurably inferior in point of magnitude. Here is a common tumbler, about three-quarters full of water. Into it I introduce two bent rods of brass, which are carefully insulated below the surface of the water by a covering of india-rubber. The points, however, are exposed, and come to within an inch of one another, near the bottom of the tumbler. Outside the tumbler, the brass rods are mounted on a stand, by means of which I can send the full charge of this Leyden jar battery through the water, from point to point. Since water is a bad conductor of electricity, as compared with metals, the charge encounters great resistance in passing through it, and in overcoming this resistance produces considerable mechanical commotion, which is usually sufficient to shiver the glass to pieces.

To charge the battery will take about twenty turns of this large Holtz machine. Observe how the pith ball of the electroscope rises as the machine is worked, showing that the charge is going in. And now it remains stationary; which is a sign that the battery is fully charged, and can receive no more. You will notice that the outside coating of the battery has been already connected with one of the brass rods dipping into the tumbler of water. By means of this discharger I will now bring the inside coating into connection with the other rod. And see, before contact is actually made, the spark has leaped across, and our tumbler is violently burst asunder from top to bottom.

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 Carré’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.

It would be easy to explode gunpowder with the electric spark, but the smoke of the explosion would make the lecture-hall very unpleasant for the remainder of the lecture. I propose, therefore, to substitute for gunpowder an explosive mixture of oxygen and hydrogen, with which I have filled this little metal flask, commonly known as Volta’s pistol. By a very simple contrivance, the electric spark is discharged through the mixture, when I hold the flask toward the conductor of the machine. A cork is fitted tightly into the neck of the flask, and at the moment the spark passes you hear a loud explosion, and you see the cork driven violently up to the ceiling.

=Destruction of Life.=--The last effect of lightning to which I shall refer, and which, perhaps, more than any other, strikes us with terror, is the sudden and utter extinction of life, when the lightning flash descends on man or on beast. So swift is this effect, in most cases, that death is, in all probability, absolutely painless, and the victim is dead before he can feel that he is struck. I cannot give you, with any degree of exactness, the number of people killed every year by lightning, because the record of such deaths has been hitherto very imperfectly kept, in almost all countries, and is, beyond doubt, very incomplete. But perhaps you will be surprised to learn that the number of deaths by lightning actually recorded is, on an average, in England about 22 every year, in France 80, in Prussia 110, in Austria 212, in European Russia 440.[25]

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.”[26]

Not less striking is the story told by Dr. Plummer, surgeon of the Illinois Volunteers, in the _Medical and Surgical Reporter_ of June 19, 1865: “Our regiment was yesterday the scene of one of the most terrible calamities which it has been my lot to witness. About two o’clock a violent thunderstorm visited us. While the old guard was being turned out to receive the new, a blinding flash of lightning was seen, accompanied instantly by a terrific peal of thunder. The whole of the old guard, together with part of the new, were thrown violently to the earth. The shock was so severe and sudden that, in most cases, the rear rank men were thrown across the front rank men. One man was instantly killed, and thirty-two men were more or less severely burned by the electric fluid. In some instances the men’s boots and shoes were rent from their feet and torn to pieces, and, strange as it may appear, the men were injured but little in the feet. In all cases the burns appear as if they had been caused by scalding-hot water, in many instances the skin being shriveled and torn off. The men all seem to be doing well, and a part of them will be able to resume their duties in a few days.”

=The Return Shock.=--It sometimes happens that people are struck down and even killed at the moment a discharge of lightning takes place between a cloud and the earth, though they are very far from the point where the flash is actually seen to pass; while others, who are situated between them and the lightning, suffer very little, or perhaps not at all. This curious phenomenon was first carefully investigated by Lord Mahon in the year 1779, and was called by him the “return shock.” His theory, which is now commonly accepted, may be easily understood with the aid of the sketch before you.

Let us suppose ABC to represent the outline of a thundercloud which dips down toward the earth at A and at C. The electricity of the cloud develops by inductive action a charge of the opposite kind in the earth beneath it. But the inductive action is most powerful at E and F, where the cloud comes nearest to the earth. Hence, bodies situated near these points may be very highly electrified as compared with bodies at a point between them, such as D. Now, when a flash of lightning passes at E, the under part of the cloud is at once relieved of its electricity, its inductive action ceases, and, therefore, a person situated at F suddenly ceases to be electrified. This sudden change from a highly electrified to a neutral state involves a shock to his system which may be severe enough to stun or even to kill him. Meanwhile, people at _D_, having been also electrified to some extent by the influence of the thundercloud, must in like manner undergo a change in their electrical condition when the flash of lightning passes, but this change will be less violent because they were less highly electrified.

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 Carré’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.

=Summary.=--Let me now briefly sum up the chief destructive effects of lightning. First, with regard to good conductors: though it passes harmlessly through them if they be large enough to afford it an easy passage, it melts and converts them into vapor if they be of such small dimensions as to offer considerable resistance. Secondly, lightning acts with great mechanical force on bad conductors; it is capable of tearing asunder large masses of masonry, and of projecting the fragments to a considerable distance. Thirdly, it sets fire to combustible materials. And lastly, it causes the instantaneous death of men and animals.

=Franklin’s Lightning Rods.=--The object of lightning conductors is to protect life and property from these destructive effects. Their use was first suggested by Franklin, in 1749, even before his famous experiment with the kite; and immediately after that experiment, in 1752, he set up, on his own house, in Philadelphia, the first lightning conductor ever made. He even devised an ingenious contrivance, by means of which he received notice when a thundercloud was approaching. The contrivance consisted of a peal of bells, which he hung on his lightning conductor, and which were set ringing whenever the lightning conductor became charged with 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.”

=Introduction of Lightning Rods into England.=--The progress of lightning conductors was more slow in England and on the Continent of Europe, owing to a fear, not unnatural, that they might, in some cases, draw down the lightning where it would not otherwise have fallen. People preferred to take their chance of escaping as they had escaped before, rather than invite, as it were, the lightning to descend on their houses, in the hope that an iron rod would convey it harmless to the earth. But the immense amount of damage done every year by lightning, soon led practical men to entertain a proposal which offered complete immunity from all danger on such easy terms; and when it was found that buildings protected by lightning conductors were, over and over again, struck by lightning without suffering any harm, a general conviction of their utility was gradually established in the public mind.

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.

For a time the Tories seemed to prevail. The king was on their side. Experiments on a grand scale were conducted in his presence, at the Pantheon, a large building in Oxford street; he was assured that these experiments proved the great superiority of balls over points; and to give practical effect to his convictions, his majesty directed that a large cannon ball should be fixed on the end of the lightning conductor attached to the royal palace at Kew. But the committee of the Royal Society remained unconvinced. In course of time the heat of party spirit abated; experience as well as reason was found to be in favor of Franklin’s views; and the battle of the balls and points has long since passed into the domain of history.[27]

=Functions of a Lightning Conductor.=--A lightning conductor fulfills two functions. First, it favors a silent and gradual discharge of electricity between the cloud and the earth, and thus tends to prevent that accumulation which must of necessity take place before a flash of lightning will pass. Secondly, if a flash of lightning come, the lightning conductor offers it a safe channel through which it may pass harmless to the earth.

These two functions of a lightning conductor may be easily illustrated by experiment. When our machine is in action, if I present my closed hand to the large brass conductor, a spark passes between them, and I feel, at the same moment, a slight electric shock. Here the conductor of the machine, as usual, holds the place of the electrified cloud; my closed hand represents, as it were, a lofty building that stands out prominently on the surface of the earth; the spark is the flash of lightning, and the electric shock just suggests the destructive power of the sudden disruptive discharge.

Now let me protect this building by a lightning conductor. For this purpose, I take in my hand a brass rod, which I connect with the earth by a brass chain. In the first instance, I will have a metal ball on the end of my lightning conductor. You see the effect; sparks pass rapidly, but I feel no shock. I can increase the strength of the discharge by hanging this condensing jar on the conductor of the machine. Sparks pass now, much more brilliant and powerful than before, but still I get no shock. It is evident, therefore, that my lightning rod does not prevent the flash from passing, but it conveys it harmless to the ground.

I next take a rod which is sharply pointed, and connecting it as before with the earth by a brass chain, I present the sharp point to the conductor of the machine. Observe how different is the result; there is no disruptive discharge; no spark passes; no shock is felt. Electricity still continues to be generated in the machine, and electricity is generated, by induction, in the brass rod, and in my body. But these two opposite electricities discharge themselves silently, by means of this pointed rod, and no sensible effect of any kind is exhibited.

These experiments are very simple, but they really put before us, in the clearest possible way, the whole theory of lightning conductors. In particular, they give us ocular demonstration that an efficient lightning rod not only makes the lightning harmless when it comes, but tends very much to prevent its coming. A remarkable example, on a large scale, of this important property, is furnished by the town of Pietermaritzburg, the capital of the colony of Natal, in South Africa. This town is subject to the frequent visitation of thunderstorms, at certain seasons of the year, and much damage was formerly done by lightning, but since the erection of lightning conductors on the principal buildings, the lightning has never fallen within the town. Thunderclouds come as before, but they pass silently over the city, and only begin to emit their lightning flashes when they reach the open country, and have passed beyond the range of the lightning conductors.[28]

But it will often happen, even in the case of a pointed conductor, that the accumulation of electricity goes on so fast that the silent discharge is insufficient to keep it in check. A disruptive discharge will then take place, from time to time, and a flash of lightning will pass. Under these circumstances, the lightning conductor is called upon to fulfill its second function, and to convey the lightning harmless to the earth.

=Conditions of a Lightning Conductor.=--From the consideration of the functions which it has to fulfill, we may now infer what are the conditions necessary for an efficient lightning conductor. The first condition is that the end of the conductor, projecting into the air, should have, at least, one sharp point. Our experiments have shown us that a pointed conductor tends, in a manner, to suppress the flash of lightning altogether; whereas a blunt conductor, or one ending in a ball, tends only to make it harmless when it comes. It is evident, therefore, that the pointed conductor offers the greater security.

But a fine point is very liable to be melted when the lightning falls upon it, and thus to be rendered less efficient for future service. To meet this danger, it has recently been suggested, by the Lightning Rod Conference, that the extreme end of the conductor should be a blunt point, destined to receive the full force of the lightning flash, when it comes; and that, a little lower down, a number of very fine points should be provided, with a view to favor the silent discharge. This suggestion, which appears admirably fitted to provide for the twofold function of a lightning conductor, deserves to be recorded in the exact terms of the official report.

“It seems best to separate the double functions of the point, prolonging the upper terminal to the very summit, and merely beveling it off, so that, if a disruptive discharge does take place, the full conducting power of the rod may be ready to receive it. At the same time, having regard to the importance of silent discharge from sharp points, we suggest that, at one foot below the extreme top of the upper terminal, there be firmly attached, by screws and solder, a copper ring bearing three or four copper needles, each six inches long, and tapering from a quarter of an inch diameter to as fine a point as can be made; and with the object of rendering the sharpness as permanent as possible, we advise that they be platinized, gilded, or nickel plated.”[29]

The second condition of a lightning conductor is, that it should be made of such material, and of such dimensions, as to offer an easy passage to the greatest flash of lightning likely to fall on it; otherwise it might be melted by the discharge, and the lightning, seeking for itself another path, might force its way through bad conductors, which it would partly rend asunder, and partly consume by fire. Copper is now generally regarded as the best material for lightning conductors, and it is almost universally employed in these countries. If it is used in the form of a rope, it should not be less than half an inch in diameter; if a band of copper is preferred--and it is often found more convenient by builders--it should be about an inch and a half broad and an eighth of an inch thick. In France it has been hitherto more usual to employ iron rods for lightning conductors, but since iron is much inferior to copper in its conducting power, the iron rod must be of much larger dimensions; it should be at least one inch in diameter.[30]

The third condition is that the lightning conductor should be continuous throughout its whole length, and should be placed in good electrical contact with the earth. This is a condition of the first importance, and experience has shown that it is the one most likely of all to be neglected. In a large town the best earth connection is furnished by the system of water-mains and gas-mains, each of which constitutes a great network of conductors everywhere in contact with the earth. Two points, however, must be carefully attended to--first, that the electrical contact between the lightning conductor and the metal pipe should be absolutely perfect; and, secondly, that the pipe selected should be of such large dimensions as to allow the lightning an easy passage through it to the principal main.

If no such system of water-pipes or gas-pipes is at hand, then the lightning rod should be connected with moist earth by means of a bed of charcoal or a metal plate not less than three feet square. This metal plate should be always of the same material as the conductor, otherwise a galvanic action would be set up between the two metals, which in course of time might seriously damage the contact. Dry earth, sand, rock, and shingle are bad conductors; and, if such materials exist near the surface of the earth, the lightning rod must pass through them and be carried down until it reaches water or permanently damp earth.

=Mischief Done by Bad Conductors.=--If the earth contact is bad, a lightning conductor does more harm than good. It invites the lightning down upon the building without providing for it, at the same time, a free passage to earth. The consequence is that the lightning forces a way for itself, violently bursting asunder whatever opposes its progress, and setting fire to whatever is combustible.

I will give you some recent and striking examples. In the month of May, 1879, the church of Laughton-en-le-Morthen, in England, though provided with a conductor, was struck by lightning and sustained considerable damage. On examination it was found that the lightning followed the conductor down along the spire as far as the roof; then, changing its course, it forced its way through a buttress of massive masonwork, dislodging about two cartloads of stones, and leaped over to the leads of the roof, about six feet distant. It now followed the leads until it came to the cast-iron down-pipes intended to discharge the rain-water, and through these it descended to the earth. When the earth contact of the lightning conductor was examined, it was found exceedingly deficient. The rod was simply bent underground, and buried in dry loose rubbish at a depth not exceeding eighteen inches. This is a very instructive example. The lightning had a choice of two paths--one by the conductor prepared for it, the other by the leads of the roof and the down-pipes--and, by a kind of instinct which, however we may explain, we must always contemplate with wonder, it chose the path of least resistance, though in doing so it had to burst its way at the outset through a massive wall of solid masonry.[31]