Part 3

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.

=Succession of Peals.=--The succession of peals produced by a single flash of lightning is due to several causes, each one of which may contribute more or less, according to circumstances, toward the general effect. First, if we accept the results arrived at by Professor Ogden Rood, of Columbia College, what appears to the eye as a single flash of lightning, consists, in fact, as a general rule, of a succession of flashes, each one of which must naturally produce its own peal of thunder; and although the several flashes, if they follow one another at intervals of the tenth of a second, will make one continuous impression on the eye, the several peals of thunder, under the same conditions, will impress the ear as so many distinct peals.

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!”

=Variations of Intensity in Thunder.=--From what has been said, it is easy to understand how the general roar of thunder is subject to great changes of intensity, during the time it lasts, according to the number of peals that may be arriving at the ear of an observer in each particular moment. But every one must have observed that even an individual peal of thunder often undergoes similar changes, swelling out at one moment with great power, and the next moment rapidly dying away. To account for this phenomenon, I would observe, first, that there is no reason to suppose that the disturbance caused by lightning is of exactly the same magnitude at every point of its path. On the contrary, it would seem very probable that the amount of this disturbance is, in some way, dependent on the resistance which the discharge encounters. Hence the intensity of the sound waves sent forth by a flash of lightning is probably very different at different parts of its course; and each individual peal will swell out on the ear or die away, according to the greater or less intensity of the sound waves that reach the ear in each successive moment of time.

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.

=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]