Part 6

=Personal Safety in a Thunderstorm.=--I suppose there is hardly any one to whom the question has not occurred, at some time or another, what he had best do to secure his personal safety during a thunderstorm. This question is of so much practical interest that I think I shall be excused if I say a few words about it, though perhaps, strictly speaking, it is somewhat beside the subject of lightning conductors.

At the outset, perhaps, I shall surprise you when I say that you would enjoy the most perfect security if you were in a chamber entirely composed of metal plates, or in a cage constructed of metal bars, or if you were incased, like the knights of old, in a complete suit of metal armor. This kind of defense is looked upon as so perfect, among scientific men, that Professor Tait does not hesitate to recommend his adventurous young friends devoted to the cause of science to provide themselves with a light suit of copper, and, thus protected, take the first opportunity of plunging into a thundercloud, there to investigate, at its source, the process by which lightning is manufactured.[36]

The reason why a metal covering affords complete protection is that, when a conductor is electrified, the whole charge of electricity exists on the outside surface of the conductor; and therefore, when a discharge takes place, it is only the outside surface that is affected. Thus, if you were completely incased in a metal covering, and then charged with electricity by the inductive action of a thundercloud, it is only the metal covering that would undergo any change of electrical condition; and when the lightning flash would pass, it is only the metal covering that would be discharged.

Let me show you a very pretty and interesting experiment to illustrate this principle: Here is a hollow brass cylinder, open at the ends, mounted on an insulating stand. On the outside is erected a light brass rod with two pith balls suspended from it by linen threads. Two pith balls are also suspended by linen threads from the inner surface of the cylinder. You know that these pith balls will indicate to us the electrical condition of the surfaces to which they are attached. If the surface be electrified, the pith balls attached to it will share in its electrical condition, and will repel each other; if the surface be neutral, the pith balls attached to it will be neutral, and will remain at rest.

I now put this apparatus under the influence of our thundercloud, that is, the large brass conductor of our machine. The moment my assistant turns the handle, the electricity begins to be developed on the conductor, and you see, at once, the effect on the brass cylinder. The pith balls attached to the outer surface fly asunder; those attached to the inner surface remain at rest. And now a spark passes; our thundercloud is discharged; the inductive action ceases; the pith balls on the outside suddenly collapse, while those on the inside are in no way affected.

It is not necessary that the brass cylinder should be insulated. To vary the experiment, I will now connect it with the earth by a chain; you will observe that the effect is precisely the same as before. Flash after flash passes while the machine continues in action; the outside pith balls fly about violently, being charged and discharged alternately; the inside pith balls remain all the time at rest. Thus you see clearly that, if you were sitting inside such a metal chamber as this, or covered with a complete suit of metal armor, you would be perfectly secure during a thunderstorm, whether the chamber were electrically connected with the earth or insulated from it.

=Practical Rules.=--But it rarely happens, when a thunderstorm comes, that an iron hut or a complete suit of armor is at hand, and you will naturally ask me what you ought to do under ordinary circumstances. First, let me tell you what you ought not to do. You ought not to take shelter under a tree, or under a haystack, or under the lee of a house; you ought not to stand on the bank of a river, or close to a large sheet of water. If indoors, you ought not to stay near the fireplace, or near any of the flues or chimneys; you ought not to stand under a gasalier hanging from the ceiling; you ought not to remain close to the gas pipes or water-pipes, or any large masses of metal, whether used in the construction of the building, or lying loosely about.

The necessity for these precautions is sufficiently evident from the principles I have already put before you. You want to prevent your body from becoming a link in that broken chain of conductors which, as we have seen, the electric discharge between earth and cloud is likely to follow. Now a tree is a better conductor than the air; and your body is a better conductor than a tree. Hence, the lightning, in choosing the path of least resistance, would leave the air to pass through the tree, and would leave the tree to pass through you. A like danger would await you if you stood under the lee of a haystack or of a house.

The number of people who lose their lives by taking refuge under trees in thunderstorms is very remarkable. As one instance out of many, I may cite the following case which was reported in the _Times_, July 14, 1887: “Yesterday the funeral of a negress was being conducted in a graveyard at Mount Pleasant, sixty miles north of Nashville, Tennessee, when a storm came on, and the crowd ran for shelter under the trees. Nine persons stood under a large oak, which the lightning struck, killing everyone, including three clergymen, and the mother and two sisters of the girl who had been buried.”

Again, every large sheet of water constitutes practically a great conductor, which offers a very perfect medium of discharge between the earth round about and the cloud. Therefore, when a thundercloud is overhead, the sheet of water is likely to become one end of the line of the lightning discharge; and if you be standing near it, the line of discharge may pass through your body.

When lightning strikes a building, it is very apt to use the stack of chimneys in making its way to earth, partly because the stack of chimneys is generally the most prominent part of the building, and partly because, on account of the heated air and the soot within the chimney, it is usually a moderately good conductor. Therefore, if you be indoors, you must keep well away from the chimneys; and for a similar reason, you must keep as far as you can from large masses of metal of every kind.

Having pointed out the sources of danger which you must try to avoid in a thunderstorm, I have nearly exhausted all the practical advice that I have at my command. But there are some occasions on which it may be possible, not only to avoid evident sources of danger, but to make special provision for your own security. Thus, for example, in the open country, if you stand a short distance from a wood, you may consider yourself as practically protected by a lightning conductor. For a wood, by its numerous branches and leaves, favors very much a quiet discharge of electricity, thus tending to suppress altogether the flash of lightning; and if the flash of lightning does come, it is much more likely to strike the wood than to strike you, because the wood is a far more prominent body, and offers, on the whole, an easier path to earth. In like manner, if you place yourself near a tall solitary tree, some twenty or thirty yards outside its longest branches, you will be in a position of comparative safety. If the storm overtake you in the open plain, far away from trees and buildings, you will be safer lying flat on the ground than standing erect.

In an ordinary dwelling house, the best situation is probably the middle story, and the best position in the room is in the middle of the floor; provided, of course, that there is no gasalier hanging from the ceiling above or below you. Strictly speaking, the _middle of the room_ would be a still safer position than the middle of the floor; and nothing could be more perfect than the plan suggested by Franklin, to get into “a hammock, or swinging bed, suspended by silk cords, and equally distant from the walls on every side, as well as from the ceiling and floor, above and below.” An interesting case has been recently recorded, by a resident of Venezuela, which illustrates in a remarkable way the excellence of this advice. “The lightning,” he says, “struck a _rancho_--a small country house, built of wood and mud, and thatched with straw or large leaves--where one man slept in a hammock, another lay under the hammock on the ground, and three women were busy about the floor; there were also several hens and a pig. The man in the hammock did not receive any injury whatever, while the other four persons and the animals were killed.”[37]

But, as I can hardly hope that many of you when the thunderstorm actually comes will find yourselves provided with a hammock, I would recommend, as more generally useful, another plan of Franklin’s, which is simply to sit on one chair in the middle of the floor and put your feet up on another. This arrangement will approach very nearly to absolute security if you take the further precaution, also mentioned by Franklin, of putting a feather bed or a couple of hair mattresses under the chairs.[38]

=Security Afforded by Lightning Rods.=--You might, perhaps, be inclined to infer hastily, from the examples I have set before you, in the course of this lecture, of buildings which were struck and severely injured by lightning though provided with lightning conductors, that a lightning rod affords a very imperfect protection to life and property. But such an idea would be entirely at variance with the evidence at hand on the subject. In all the cases to which I have referred, and in many others which might easily have been cited, the damage was done simply because the lightning rods were deficient in one or more of the conditions on which I have so much insisted. Where these conditions are fulfilled, the lightning flash will either not come down at all upon the building, or, if it do come, it will be carried harmless to the earth.

Perhaps there is no one fact that so forcibly brings home to the mind the complete protection afforded by lightning conductors as the change which followed their introduction into the Royal Navy. I have already told you that in former times the damage done by lightning to ships of the Royal Navy was a regular source of expenditure, amounting every year to several thousand pounds sterling. But, after the general adoption of lightning conductors about forty years ago, through the indefatigable exertions of Sir William Snow Harris, this source of expenditure absolutely disappeared, and injury to life and property has long been practically unknown in Her Majesty’s Fleet.

I should say, however, that the trial of lightning conductors in the Navy, though it lasted long enough to prove their perfect efficiency, has almost come to an end in our own days. The great iron monsters which in recent times have taken the place of the wooden ships of Old England are quite independent of lightning rods in the common sense of the word. Their ponderous masts are virtually lightning rods of colossal dimensions, and their unsightly hulls are, so to speak, earth-plates of enormous size in perfect electrical contact with the ocean. To add to such structures lightning conductors of the common kind would be nothing better than “wasteful and ridiculous excess.”

As regards buildings on land, I may refer to the little province of Schleswig-Holstein, of which I have already spoken to you. From some cause or other this small peninsula is singularly exposed to thunderstorms, and of late years it has been more abundantly provided with lightning conductors than, perhaps, any other district of equal extent in Europe. Now, as a simple illustration of the protection afforded by these lightning conductors, I may mention that, on the 26th of May, 1878, a violent thunderstorm burst over the little town of Utersen. Five several flashes of lightning fell in different parts of the town, but not the slightest harm was done, each flash being safely carried to earth by a lightning conductor. Further, it appears from the records of the fire insurance company that, out of 552 buildings injured by lightning during a period of eight years--from 1870 to 1878--only four had lightning conductors; and in these four cases it was found, on examination, that the lightning conductors were defective.[39]

It would be easy to multiply evidence on this subject. But as I have already trespassed, I fear, too far on your patience, I will content myself with saying, in conclusion, that according to all the highest authorities, both practical and theoretical, any structure provided with a lightning conductor properly fitted up in conformity with the principles I have set before you to-day is perfectly secure against lightning. The lightning, indeed, may fall upon it, but it will pass harmless to the earth; and the experience of more than a hundred years has fully justified the simple and modest words of the great inventor of lightning conductors: “It has pleased God, in His goodness to mankind, at length to discover to them the means of securing their habitations and other buildings from mischief by thunder and lightning.”

NOTE I.

ON THE LIGHTNING CONDUCTOR AT BEREHAVEN.[40]

It is satisfactory to know that the lightning conductor referred to in my lecture as attached to the lighthouse at Berehaven has been put in good order under the best scientific guidance. The following interesting letter from Professor Tyndall, which appeared in the _Times_, August 31, 1887, gives the history of the matter very clearly, and fully bears out the views put forward in my lecture:

“Your recent remarks on thunderstorms and their effects induce me to submit to you the following facts and considerations. Some years ago a rock lighthouse on the coast of Ireland was struck and damaged by lightning. An engineer was sent down to report on the occurrence; and, as I then held the honorable and responsible post of scientific adviser to the Trinity House and Board of Trade, the report was submitted to me. The lightning conductor had been carried down the lighthouse tower, its lower extremity being carefully embedded in a stone perforated to receive it. If the object had been to invite the lightning to strike the tower, a better arrangement could hardly have been adopted.

“I gave directions to have the conductor immediately prolonged, and to have added to it a large terminal plate of copper, which was to be completely submerged in the sea. The obvious convenience of a chain as a prolongation of the conductor caused the authorities in Ireland to propose it; but I was obliged to veto the adoption of the chain. The contact of link with link is never perfect. I had, moreover, beside me a portion of a chain cable through which a lightning discharge had passed, the electricity in passing from link to link encountering a resistance sufficient to enable it to partially fuse the chain. The abolition of resistance is absolutely necessary in connecting a lightning conductor with the earth, and this is done by closely embedding in the earth a plate of good conducting material and of large area. The largeness of area makes atonement for the imperfect conductivity of earth. The plate, in fact, constitutes a wide door through which the electricity passes freely into the earth, its disruptive and damaging effects being thereby avoided.

“These truths are elementary, but they are often neglected. I watched with interest some time ago the operation of setting up a lightning conductor on the house of a neighbor of mine in the country. The wire rope which formed part of the conductor was carried down the wall and comfortably laid in the earth below without any terminal plate whatever. I expostulated with the man who did the work, but he obviously thought he knew more about the matter than I did. I am credibly informed that this is a common way of dealing with lightning conductors by ignorant practitioners, and the Bishop of Winchester’s palace at Farnham has been mentioned to me as an edifice ‘protected’ in this fashion. If my informant be correct, the ‘protection’ is a mockery, a delusion, and a snare.”

NOTE II.

BOOKS OF REFERENCE.

As some of my readers may wish to pursue the study of lightning and lightning conductors beyond the limits to which a popular lecture must, of necessity, be confined, I subjoin a list of the books which I think they would be likely to find most useful for the purpose. Among ordinary text-books on physics--Jamin, Cours de Physique, vol. i., pp. 470-494; Mascart, Traité d’Electricité Statique, vol. ii., pp. 555-579; De Larive, A Treatise on Electricity, in three volumes, London, 1853-8, vol. iii., pp. 90-201; Daguin, Traité de Physique, vol. iii., pp. 209-280; Riess, Die Lehre von der Reibungs-Elektricität, vol. ii., pp. 494-564; Müller-Pouillet, Lehrbuch der Physik, Braunschweig, 1881, vol. iii., pp. 210-225; Scott, Elementary Meteorology, chap. x. Of the numerous special treatises and detached papers on the subject, I would recommend Instruction sur les Paratonnerres adopté par l’Académie des Sciences, Part i., 1823, Part ii., 1854, Part iii., 1867, Paris, 1874; Arago, Sur le Tonnerre, Paris, 1837; also his Meteorological Essays, translated by Sabine, London, 1855; Sir William Snow Harris, On the Nature of Thunderstorms, London, 1843; also by the same writer, A Treatise on Frictional Electricity, London, 1867; and various papers on lightning conductors, from 1822 to 1859; Tomlinson, The Thunderstorm, London, 1877; Anderson, Lightning Conductors, London, 1880; Holtz, Ueber die Theorie, die Anlage, und die Prüfung der Blitzableiter, Greifswald, 1878; Weber, Berichte über Blitzschläge in der Provinz Schleswig-Holstein, Kiel, 1880-1; Tait, A Lecture on Thunderstorms, delivered in the City Hall, Glasgow, in 1880, Nature, vol. xxii.; Report of the Lightning Rod Conference, London, 1882. This last-mentioned volume comes to us with very high authority, representing, as it does, the joint labors of several eminent scientific men selected from the following societies: The Meteorological Society, the Royal Institute of British Architects, the Society of Telegraph Engineers and Electricians, the Physical Society.

Since the above was in print, two lectures given before the Society of Arts by Professor Oliver Lodge, F. R. S., have appeared in the _Electrician_, June and July, 1888, in which some new views are put forward respecting lightning conductors, that seem deserving of careful consideration.

FOOTNOTES:

[17] The Thunderstorm, by Charles Tomlinson, F. R. S., Third Edition, pp. 153-4.

[18] Two Lectures on Atmospheric Electricity and Protection from Lightning, published at the end of his Treatise on Frictional Electricity, p. 273.

[19] See Report of Lightning Rod Conference, p. 119.

[20] _Loco citato._

[21] Sir William Snow Harris, _loco citato_, p. 274.

[22] _Id._, p. 275.

[23] The Thunderstorm, by Charles Tomlinson, F.R.S., Third Edition, p. 172.

[24] See for these facts, Anderson, Lightning Conductors, p. 197; Tomlinson, The Thunderstorm, pp. 167-9; Harris, _loco citato_, pp. 273-4.

[25] See Anderson, Lightning Conductors, pp. 170-5.

[26] The Thunderstorm, pp. 158-9. See also an account of four persons who were struck on the Matterhorn, in July, 1869, all of whom were hurt, and none killed: Whymper’s Scrambles Among the Alps, pp. 414, 415.

[27] See Philosophical Transactions of the Royal Society, 1773, p. 42, and 1778, part i., p. 232; Anderson’s Lightning Conductors, pp. 40-2; Lighting Rod Conference, pp. 76-9.

[28] See A Lecture on Thunderstorms, by Professor Tait of Edinburgh, published in Nature, vol. xxii., p. 365.

[29] Report of the Lightning Rod Conference, p. 4.

[30] The dimensions here set forth are greater in some respects than those “recommended as a minimum” in the report of the Lightning Rod Conference, page 6. But it will be observed by those who consult the report that the minimum recommended is just the size which, in the preceding paragraph of the report, is said to have been actually melted by a flash of lightning; and, therefore, it seems not to be a very safe minimum. It will be also seen that there is some confusion in the figures given, and that they contradict one another. For the dimensions of iron rods, see the instructions adopted by the Academy of Science, Paris, May 20, 1875; Lightning Rod Conference, pp. 67-8.

[31] See letter of Mr. R. S. Newall, F. R. S., in the _Times_, May 30, 1879.

[32] See Nature, June 12, 1879, vol. xx., p. 146.

[33] See letter of Mr. Tomes in Nature, vol. xx., p. 145; also Lightning Rod Conference, pp. 210-15.

[34] See Anderson, Lightning Conductors, pp. 208-10.

[35] See Lightning Rod Conference, pp. 208-10; see also the note at the end of this Lecture, p. 52.

[36] Lecture on Thunderstorms, Nature, vol. xxii., pp. 365, 437. See, also, a very interesting paper by the late Professor J. Clerk Maxwell, read before the British Association at Glasgow in 1876, and reprinted in the report of the Lightning Rod Conference, pp. 109, 110.

[37] Nature, vol. xxxi., p. 459.

[38] See further information on this interesting subject in the Report of the Lightning Rod Conference, pp. 233-5.

[39] See “Die Theorie, die Anlage, und die Prüfung der Blitzableiter,” von Doctor W. Holtz, Griefswald, 1878.

[40] See page 44.

APPENDIX.

RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.

The lecture on lightning conductors contained in this volume fairly represents, I think, the theory hitherto received on the subject. It is, moreover, entirely in accord with the report of the Lightning Rod Conference, brought out in 1883, by a committee of most eminent men, representing several branches of science, who were specially chosen to consider this question some ten years ago.

=Lectures of Professor Lodge.=--But, in the month of March, 1888, two lectures were given before the Society of Arts, in London, by Professor Oliver Lodge, in which this theory was directly challenged, and attacked with cogent arguments, supported by striking and original experiments. These lectures gave rise to an animated controversy, which culminated in a formal discussion at the recent meeting of the British Association in Bath. The discussion was carried on with great spirit, and most of the leading representatives of physical and mechanical science took an active part in it. The greater portion of this volume was printed off before the meeting of the British Association took place. But the discussion on the theory of lightning conductors seemed to me so interesting and important that I thought it right, in the form of an Appendix, to give some account of the questions at issue, and of the opinions expressed upon them.

Professor Lodge maintains[41] that the received theory of lightning rods is open to two objections. First, it takes account only of the conducting power of the lightning rod, and takes no account of the phenomenon known as self-induction, or electrical inertia. Secondly, it assumes that the whole substance of a lightning rod acts as a conductor, in all cases of lightning discharge; whereas there is reason to believe that, in many cases, it is only a thin outer shell that really comes into action. I will deal with these two points separately.