Chapter XIV: Appendix: B.—as to Solid or Hollow Conductors. by Sir W. Snow Harris,

F.R.S.

1. A given quantity of electricity melts the same quantity of metal, whether in a solid or hollow form. So far it is immaterial which form the conductor has. But supposing the mass of metal to be so large that the heating effect may be neglected. It is proved that the greater the surface, the less is its intensity or power at any point, the intensity approaching the second power or square of the surface inversely. It is important to give the charge free room of expansion by increasing the surface of conductor, so as to reduce the mechanical activity of shock to the least possible. Rectangular flat bars may be employed.

2. A rain water pipe communicating with main conductor, should have earth connection. All imperfect substances, as masonry, and ship masts, transmit a certain portion of electricity without explosive action. One great use of the conductor is to relieve the wood or masonry of the quantity it cannot discharge without explosion.

3. Conductors of small iron rod or wire are very objectionable. They commonly rust at the joints, and have fallen to pieces, and often been knocked to pieces by lightning. Iron may, certainly, be employed with advantage, but should be galvanized. Zinc is an even better conductor than iron; and being spread over the surface is not open to the objection of making a conductor of two metals of unequal conducting power. A good and efficient conductor might be formed of galvanized iron. It should be of wrought iron, galvanized, of 2 inches diameter, with screwed joints of _extra_ thickness. Copper tubing is, however, always to be preferred.

4. In dry or rocky soil, complete the conductor by leading old iron chains out from the walls in several directions, or by leading a flow of water over them. Fortunately a thunder storm is usually attended by heavy rains. The iron chains should extend 30 to 50 feet, and be a foot or 18 inches under ground. The termination in a large surface of moist earth would be preferable to that in a well, as the action is a superficial one of expansion in all directions. In the _tin_ leaf coatings of the electrical jar, the charge is not influenced by the thickness of metal.

W. SNOW HARRIS.

REPORT ON THE DESTRUCTION BY LIGHTNING OF A GUNPOWDER STORE AT
BRUNTCLIFFE, YORKSHIRE. BY MAJOR Y. D. MAJENDIE, R.A.

(_Abstracted by G. J. Symons, F.R.S._)

The Gunpowder exploded at 4.30 p.m. on August 6th, 1878, during the greatest intensity of a violent thunderstorm. The building, was brick, with brick arched roof, length 9 feet, width 5 feet, height 6 feet (internal dimensions). The store had a uniform thickness of three bricks, and was furnished at the one end with an iron door, at the other end with a lightning conductor. The conductor consisted of a copper wire rope, 10 gauge copper wire, the rope being 7/16 inch thick, having four points at the top (one large one in the centre, and three smaller ones round it), it extended to about 13 feet above the top of the building, and about the same length was carried into the ground and terminated in a drain. The conductor had been erected in 1876, by Mr. John Bisby, of Leeds, and was fixed to a pole distant about 2 inches from the end of the building opposite to that in which the iron door was fixed, it was not connected with the iron door in any way. No one was near the store when the powder exploded, and it seems probable that the earth connection of the conductor was bad, that the mass of iron in the door offered at least an equally good path—and that the gunpowder was ignited by a flash passing between, the two imperfect conductors.

“The only structural damage effected was produced by the impingement of bricks, which striking with great force, had in a few instances, partially penetrated or displaced brick work in the dwelling-houses and buildings, and a portion of the iron of an iron church was broken by a piece of projected _débris_. A brick was driven through a window in one of the houses at three hundred yards, and broke a bedstead. As far as I have been able to discover no other structural injury was occasioned.”

This accident appears to suggest several conclusions:—

“In the first place it appears to me to afford a striking confirmation of the principle which has been repeatedly and emphatically enunciated by Sir William Snow Harris and other authorities on the subject of lightning conductors, that in order to secure an efficient protection for a given building, all the metal of the building, and as far as possible the whole of the structure itself, should be brought into actual connection with the system of conduction; in other words, that the general conducting power of the mass of the edifice should be completed, and all attractive and prominent parts allied in one protective combination, so as to “bring the whole” (as it has been expressed by Sir William Snow Harris) “as nearly as may be into that passive or non-resisting state which it would assume, supposing the whole were a mass of metal.” In the present case, assuming the conductor itself to have been efficient, a point which there seems no sufficient reason for doubting, the system of conduction was obviously defective. Not only was the whole length of the building left unprotected, the conductor having been on a pole at one end, and carefully insulated from the building, but the iron door which was at the opposite end, was absolutely unconnected therewith, and was not itself supplied with any earth connection.”

“It appears clear, therefore, that even what may be deemed _per se_ an efficient lightning conductor, _i.e._ a conductor, which considered alone, offers a path of little or no resistance even to a powerful electric current, does not afford a reliable protection to a building unless it be scientifically applied, and with due regard to those principles upon which the more eminent authorities on electrical science are agreed. To a disregard of these principles, especially in respect of the iron door being left out of the system of conduction, and unconnected therewith, I believe the present accident may be attributed.”

REPORTS OF COMMITTEES ON THE POWDER MAGAZINES AT PURFLEET.

(Phil. Trans., 1773, p. 42, and 1778, Part I., p. 232.)

(_Abstracted by Prof. W. G. Adams, F.R.S._)

Report of a Committee consisting of the Hon. Henry Cavendish, Dr. Watson, Dr. Franklin, Mr. J. Robertson, Mr. Wilson, and Mr. Delaval, appointed by the Royal Society, “to consider of a method for securing the powder magazine at Purfleet.”

A powder mill at Brescia having blown up in consequence of being struck by lightning, the Board of Ordnance applied to Mr. B. Wilson to know in what way the powder magazine could be protected. He recommended that a blunt conductor should be employed, whereas Dr. Franklin recommended a pointed conductor. The Committee met and Dr. Franklin read a paper on the subject, and the report of the Committee was in conformity with Dr. Franklin’s views.

The Committee went to Purfleet and examined the buildings. They found that the barrels of powder, when the magazines were full, lay piled on each other up to the spring of the arches; on each barrel were four copper hoops, which with vertical iron bars formed broken conductors within the building. These iron bars were ordered to be removed.

The Committee advised that at _each end_ of each magazine a well should be dug in or through the chalk, so deep as to have in it at least four feet of standing water. From the bottom of this water should arise a piece of leaden pipe to or near to the surface of the ground, where it should be strongly joined to the end of an upright iron bar, an _inch and a half_ in diameter, fastened to the wall by leaden straps, and extending ten feet above the ridge of the building, tapering from the ridge upwards to a sharp point, the upper twelve inches of copper, the iron to be painted.

Lead was mentioned for the underground part as less liable to rust, in the form of a pipe as giving greater stiffness for the substance, and iron for the part above ground as stronger, and less likely to be cut away. The pieces of which the bar may be composed should be screwed strongly into each other by a close joint with a thin plate of lead between the shoulders. Each rod in passing above the ridge should be strongly and closely connected by iron or lead, _or both_, with the leaden coping of the roof, so making metallic communication between the two bars of each building.

It was also advised that two wells be dug within twelve feet of the doors, one to the north of the north building and the other to the south of the south building, and that metallic communications be made between the water in them and the leaden coping of the roof.

The Board house stood 150 yards from the magazines, on elevated ground, and was a “lofty building with a pointed hip-roof, the copings of lead down to the gutters, from which leaden pipes descend at each end of the building into the water of wells of forty feet deep, for the purpose of conveying water forced up by engines to a cistern in the roof.”

As to the Board-house, they thought it already well furnished with conductors by the several leaden communications above-mentioned from the point of the roof down into the water, and that by its height and proximity it may be some security to the building below it; they therefore proposed no other conductor for that building, and only advised erecting _a pointed iron rod_ on the summit, similar to those before described and communicating with those conductors.

Mr. Wilson dissented from that part of the Report which recommended that each conductor should be pointed, because, _he says_, “by points we solicit the lightning, and may promote the mischief by drawing the charges from charged clouds, which would not discharge at all on the building if there were no points on the conductors.” By experiments made and appealed to at the Committee the difference in the effects between pointed and blunted conductors is as twelve to one. Mr. Wilson states that, “A thunder cloud, therefore, if it acted at 1200 yards distance upon a point, would require a blunted end to be brought within the distance of 100 yards, and beyond those limits would pass over it without affecting it at all.” He also says, “The _longer_ the conductors are above the building, the more danger is to be apprehended from them. I have always considered pointed conductors as being unsafe by their great readiness to collect the lightning in too powerful a manner.”

Mr. Wilson adds an account of an accident to St. Paul’s Church, and some curious reasoning on it in support of his own views. (See Phil. Trans. 1773, p. 59–61.)

* * * * *

On the 15th of May, 1777, the Board House at Purfleet was struck by lightning, and some of the brickwork damaged (See Phil. Tran., 1778, Pt. I., p. 232). About 6 p.m., after heavy rain through the day, a heavy cloud hung over the house for some time, and Mr. Nickson, who watched it from the house and gives the account, says he suspected that some of the conductors might find employment from it. He had not been long at the window before a violent flash of lightning and clap of thunder came together. The lightning struck one of the iron cramps that hold the coping, and made a dent in the lead of the cramp and the stone adjoining it, throwing some stone down and slightly disturbing about a cubic foot of brickwork at A. The iron cramp was situated over a plate of lead, and the ends of it, inserted in the stone, came within 7 inches of that plate, which communicated with the gutter, and served as a fillet to it; this gutter was part of the main conductor of the building. The lightning struck through the stone, &c., to the corner of the plate, fusing a very small portion of it. From this point no farther effect of the lightning could be traced. At the distance of seven feet and a-half from the place struck, a large leaden pipe went down from the gutter to a cistern of water in the yard. It is remarkable that the surface of one of the hip-rafters, four inches and a-half in diameter, covered with lead (communicating with the gutter), and _reaching within twenty-eight inches_ of the place struck, seems not to have been at all affected. The distance from the point of the conductor on the house to the part struck was forty-six feet.

A fresh Committee of the Royal Society, consisting of Mr. Henly, Mr. Lane, Mr. Nairne, and Mr. Planta, recommended a channel to be made from cramp to cramp round the parapet, filled with lead, and connected in four places with the main conductor on the roof of the building.

Mr. Wilson again dissented from their report, and attributed the hanging of a heavy cloud over the house (it being calm at the time) to the presence of the pointed lightning conductor.

An account of Mr. Wilson’s elaborate series of experiments at the Pantheon on a long cylinder to illustrate the effects of pointed and rounded conductors occupies seventy pages of the Philosophical Transactions; and another Committee of the Royal Society, consisting of Sir John Pringle, Dr. Watson, Henry Cavendish, W. Henly, Bishop Horsley, T. Lane, Lord Mahon, E. Nairne, and Dr. Priestley, report in favour of having additional conductors ten feet high, with copper eighteen inches long, finely tapered and acutely pointed placed upon the magazines. They conclude that “elevated rods are preferable to low conductors terminated in rounded ends, knobs, or balls of metal,” conceiving that, the experiments and reasons, made and alleged to the contrary by Mr. Wilson, are inconclusive.

Mr. Wilson’s objections are again urged by Dr. Musgrave, but called in question by Mr. Nairne (see Phil. Trans., 1778, Pt. 2, p. 823), who makes a series of experiments to illustrate the advantage of pointed conductors.

Both Mr. Wilson’s and Mr. Nairne’s experiments agree in showing that “pointed conductors draw off the electricity from a cloud at a much greater distance than those which are blunted.” Mr. Wilson objecting that this draws the charged cloud from a greater distance; and Mr. Nairne concluding that “a charged body is exhausted of more of the fluid by a pointed than by a blunted conductor,” and so is not likely to cause so much damage since it discharges itself more gradually.

EXPERIMENTS AND OBSERVATIONS ON ELECTRICITY.

BY BENJAMIN FRANKLIN. Fifth edition. London, 1774.

(_Abstracted by Prof. T. Hayter Lewis, F.S.A._)

The author shows that pointed bodies draw off electricity much more effectually than blunt ones.

When the land is hot, “the lower air is rarified and rises; the cooler, denser air above, descends.”

The clouds meet over the heated place, “and if some are electrified and others not, lightning and thunder succeed, and showers fall.”

“As electrified clouds pass over a country, high hills and trees, towers, spires, masts, chimneys, &c., as so many points, draw the electrical fire and the whole cloud discharges there.”

Therefore it is dangerous to take shelter under a tree. It is safer to be in the open fields, especially if the clothes are wet.

Metals are fused, possibly without heat; the lightning creating a violent repulsion of the particles of the metal it passes through.

[He afterwards admits this opinion to be erroneous.]

Describes experiments with sharp-pointed metallic bodies, and says: “May not the knowledge of this power of points be of use to mankind in preserving houses, churches, ships, &c., from the stroke of lightning, by fixing on their highest parts upright rods of iron, made sharp as a needle, and gilt, to prevent rusting; and from the feet of these rods lead iron wire down the outside of the building into the ground; or down one of the shrouds of a ship and her side till it reaches the water.”

“Would not pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief?”

He mentions the case of the topmast heads of a ship being struck, but having flames upon them like very large torches before the stroke.

He thinks that if there had been a good wire conductor from the heads to the sea there would have been no stroke or damage.

He records the experiments on the 10th of May, 1752, at Marly, of M. D’Alibard, who placed upon an electrical body a pointed bar of iron 40 feet high. In a thunder storm sparks of fire were attracted from it.

Again, at Paris, on the 18th of May, with the same result, by M. de Lor, with a bar of iron 99 feet high upon a cake of resin 3 inches thick and 2 feet square.

Similarly in London in July, 1752, by Mr. Canton.

He refers to other experiments.

He experimented in 1752 with a kite of thin silk (as being able to bear the wet), having a very sharp-pointed wire fixed to its top, above which it rose about a foot. The kite was raised by twine, the part in the hand being made of silk and kept quite dry.

The pointed wire will draw the electric fire from thunder clouds, and when the rain has wet (_sic_) the kite and twine, so that it conducts the electric fire freely, they will be electrified, and the electric fire will stream out plentifully on the approach of the knuckle.

“Spirits may be kindled, &c., as with a rubbed glass or tube, and thereby the sameness of the electric matter with that of the lightning be completely demonstrated.”

September, 1752. He erected “an iron rod to draw the lightning into his house in order to experiment on it.”

After many experiments, he concluded that “the clouds of a thunderstorm are most commonly in a negative state of electricity, but sometimes in a positive state.” The latter, he believed, rare.

“So that, for the most part, in thunderstrokes, it is the earth that strikes into the clouds, and not the clouds into the earth.”

In the contrary (rare) case the cloud was, “I conjecture, compressed by the driving winds or some other means, so that part of what it had absorbed was forced out, and formed an electric atmosphere round it in its denser state, so communicated positive electricity to my rod.”

“The electric fluid, moving to restore the equilibrium between the cloud and the earth, takes, in its way, all the conductors it can find (_v._ page 132 of Franklin’s book)—as metals, damp walls, moist wood, &c.—and will go considerably out of a direct course for the sake of the assistance of a good conductor.”

“Explosions only happen when the conductors cannot discharge it as fast as they receive it, by reason of their being incomplete, disunited, or too small, or not of the best materials for conducting.”

He supposes that a wire ¼ inch diameter will conduct the electricity of any one stroke of lightning ever known.

Iron is the best material, as least liable to fuse.

“Pointed rods erected on buildings and communicating with moist earth would either prevent a stroke, or, if not prevented, would conduct it so that the building should suffer no damage.

He gives instances of a small wire acting as conductor and saving the building, though the wire, being too small, was utterly destroyed.

His theory as to the crooked course of lightning is as follows:

“Who knows but that there may be, as the ancients thought, a region of this fire (electric) above our atmosphere, prevented by our air and its own too great distance of attraction from joining our earth. Yet some of it be low enough to attach itself to our highest clouds,” which thence become electrified, &c.

“I am still at a loss about the manner in which clouds become charged with electricity, no hypothesis I have yet formed perfectly satisfying me.”

He describes how he and others have been struck down by electric shocks without feeling pain or sustaining permanent injury.

For protecting powder magazines, erect a mast not far from it, and 15 or 20 feet above the top of it, with a thick iron rod fastened to it, reaching down till it comes to water.

“In buildings the rod may be fastened to the walls, chimneys, &c., with staples of iron. The lightning will not leave the rod (a good conductor) to pass into the wall (a bad conductor) through these staples. It would rather, if anywhere in the wall, pass out of it into the rod to get more readily into the earth.”

If the building be very extensive, two or more rods may be placed at different parts for greater security.

It is well not to sit near the chimney, or gilt objects, during a thunderstorm.

AN ESSAY ON THE CAUSE OF LIGHTNING, _and the manner by which the thunder clouds become possessed of their electricity, deduced from known facts and properties of that matter, to which are added plain directions for constructing and erecting safe conductors. By_ JOHN SIMMONS. 8vo. 1775.

(_Abstracted by Prof. T. Hayter Lewis, F.S.A._)

“As on the earth the operation necessary for the excitation and collection of the electric fluid is attrition.” ... “So we may rationally conclude that attrition is the means of excitation and collection of electric matter in the clouds as well as on the earth.”

By metallic conductors buildings may be preserved from the effects of lightning

Electricity ascends from the earth to the clouds by means of moist air.

“A conductor is a continuation of metal from a certain height above the highest part of a building to moist earth or water” ... “for easy and safe passage of lightning.”

Metal is the best of all conductors.

The author quotes from Franklin “buildings that have their roofs covered with lead and spouts of lead continued from roof into ground to carry off the water, are never hurt by lightning when it falls on such a building.”

The conductor may be made of any metal, and flat or round.

But nowhere less than ¾ inch diameter except at terminal.

But iron rusts, so copper or lead should be used. Lead is best, used in strips 4 inches wide and ⅒th inch thick.

Good earth contact required in moist earth (going therein at least 5 feet) or water.

The several lengths of the conductor must be well in contact by being screwed, if of iron; soldered, if of lead.

The upper terminal to be iron or copper rod 9 or 10 feet long, ¾ inch diameter, and 2 to 5 feet above top of highest chimney or other part of building.

It should be pointed as this attracts electricity better.

Lead roofs to be connected with conductor. (Examples given of house and ship struck.)

No building or object is known to have been struck by lightning within 50 feet of a proper conductor. But a tree has been shivered within 52 feet, so we may conclude that protecting influence extends to 50 feet horizontally in every direction from the point of conductor.

In gunpowder stores, conductors are not to be fixed to the buildings, but at (say) 12 feet away, fastened to a standard, the top being as high above the building as it can be conveniently.

No metal on sides or roof of the building is to be exposed to the lightning so as to attract it.

A TREATISE on ATMOSPHERIC ELECTRICITY.

BY JOHN MURRAY. 1830.

(_Abstracted by Prof. W. G. Adams, F.R.S._)

In Chapter V., on lightning identified with electricity, the author speaks of fire-balls and the Aurora Borealis, and ascribes the formation of shooting stars to electrical action. He does not believe they come from distant space into our atmosphere, but regards them as concretions formed by a flash of lightning darting through gaseous media and atmospheric air _expanded by heat_, carrying metallic dust and earthy particles ejected from volcanoes, or carried up by _evaporation_ or other causes, and diffused over an immense surface in the upper regions of the air. “The lightning carries, like a _ploughshare_, the accumulated matter in its progress, and, by the powerful electrical attraction thus excited, these particles will be drawn into the vortex of the lightning instantaneously; for, the lightning finally encountering an electricity of an opposite kind, an explosion ensues, and the collected mass is instantaneously fused and agglutinated, while the meteorite thus formed tumbles to the ground.... We therefore do not see the necessity of considering meteoric stones _extra atmospheric_.”

In this way John Murray goes on page after page, but the above will probably be sufficient notice of his work.

The following are the conditions he lays down for a good conductor:—

1. A finely pointed summit to offer an unresisting entrance.

2. A sufficient length to anticipate, as it were, the descending electricity, and receive it on its summit before it could reach any part of the building.

3. A superior conducting power in the material of the rod to facilitate its passage to the earth.

4. A sufficient thickness to prevent its fusion, which, however, will greatly depend on the resistance it has encountered in entering the conductor. And, finally

5. A safe conduction to a well or moist surface below ground.

He says: “Let the wires below ground in contact with moisture pass through a cylinder of zinc before they diverge to form the root, the copper wires will in this case always remain free from any oxidation.”

HARRIS’S LIGHTNING CONDUCTORS. REPORT _to the Committee upon_ MR. SNOW
HARRIS's _and other_ LIGHTNING CONDUCTORS.

(February 11th, 1840. Parliamentary Paper. Fcap. folio).

(_Abstracted by Professor W. E. Ayrton_).

Instances are given of ships not provided with lightning conductors being struck and damaged, whilst others lying near, and provided with conductors, were not injured. The question of lightning conductors attracting lightning considered, and evidence shown to the contrary. Lateral discharge from a lightning conductor considered. Evidence against it, if only the conductor were continuous and of sufficient size. Faraday considered that a man leaning against one of Harris’s conductors when the electricity descended would not be hurt. Proposition to place a globe of glass on the head of the mast in place of a lightning conductor considered, and the conclusion arrived at that it would do harm.

Wheatstone stated that “in the Report of the Committee of the Academy of Sciences of Paris, appointed to investigate the utility of lightning conductors, there is no instance on record of an iron rod of ½ inch in diameter being fused or even made red-hot by a flash.”

Mechanical objections to lightning conductors on ships considered and discussed. Decided that the application of Mr. Harris’s conductor tended rather to strengthen than weaken the mast and spars. Then follows a large number of letters, giving accounts of accidents from lightning to ships, &c.

Decision arrived at that on the whole Mr. Harris’s conductor is the best of those examined.

THE DIFFERENCE BETWEEN LEYDEN DISCHARGES AND LIGHTNING FLASHES. BY C. V.
WALKER, Hon. Sec. Lon. Electrical Soc. London. 1842.

(_Abstracted by Prof. T. Hayter Lewis, F.S.A._)

The author alludes to the experiments of Franklin, &c.

The distance to the lower surface of clouds, observed by Le Gentil and others, shows an average of 1000 to 2000 feet, whereas the greatest length of spark with a large machine is 3 to 4 feet.

The inductive action bears some inverse ratio to the distance.

Leaves of trees have a remarkable property of silently drawing off electricity.

He gives the particulars of a large number of experiments, with arguments thereon, to prove the theory of the difference between Leyden discharges and lightning.

Quotes examples of lightning on conductors and buildings to show that the conductor takes part only of the charge, the remainder taking other paths. Contiguous semi-insulated bodies must not be left unconnected with the lightning rod.

He quotes, with approval, the advice of Faraday, viz., to tie together with a metallic connection all contiguous readily-conducting bodies.

Cites numerous other opinions to the same effect, viz., that all metallic parts of a building should be connected with the conductor.

He sums up by stating “that the Leyden charge differs considerably not so much in _nature_ as in _degree_ from that of the cloud, inasmuch as the proximity of the coatings in the one case is infinitely small compared with the distance in the other,” &c.

He expresses great confidence in Sir W. S. Harris’s system for protecting ships.

THE EFFECT OF A LIGHTNING FLASH ON THE STEEPLE OF BRIXTON CHURCH, AND OBSERVATIONS ON LIGHTNING CONDUCTORS GENERALLY. BY C. V. WALKER. London. 1842.

(_Abstracted by Prof. T. Hayter Lewis, F.S.A._)

The author refers to Faraday’s experiments, as shewing instances of lateral discharge, and says, “unless precautions are taken to prevent its proceeding from a lightning conductor, that instrument literally invites the enemy within doors.”

He gives detail of the accident at Brixton, there being no lightning conductor.

The stroke did much damage to the steeple and then passed off harmlessly by the metal gutters and rain-water pipes.

One side of the steeple was drenched with wet and carried off part of the stroke.

He quotes examples of the apparently protective action of high trees.

Lofty trees near lofty buildings would materially mitigate, if not prevent, the violence of the stroke.

The accident at Brixton shows that the lightning takes not simply the _shortest_, but, in addition, the _largest_ path.

Had the steeple been provided with a lightning conductor outside, passing near the clock face or the bells, or water pipe, it is more than probable that a flash would pass from it to these vicinal conductors.

If _outside_ the tower the danger would be greater. He recommends that the _metal_ cross on the steeple be _replaced_ by a _stone_ one, and that the present iron rain water-pipes be connected by copper rods or plates, which are also to be connected with the lead work of roof.

The bells are also to be connected with each other and with the conductor.

Every bolt-clamp or other piece of metal within “striking distance” of the conductor, unless in direct communication with it, is liable to cause lateral discharge.

The odour developed by lightning was, at Brixton, decidedly sulphurous, as a piece of stone which was shattered by the stroke retained the odour of sulphur distinctly for several hours.

ON THE NATURE OF THUNDERSTORMS; AND ON THE MEANS OF PROTECTING BUILDINGS AND SHIPPING AGAINST THE DESTRUCTIVE EFFECTS OF LIGHTNING. BY W. SNOW HARRIS, F.R.S. 1843.

(_Abstracted by Prof. Ayrton._)

_The backstroke may do injury_, that is, a person may be killed in consequence of a flash of lightning passing between the clouds and the earth at some distance from the person.

In the Phil. Trans. for 1787, Mr. Brydone writes to the President of the Royal Society, and mentions the case of two men riding in two carts, the front one drawn by two horses, these horses and the man driving them were killed;, the man on the hinder cart and a shepherd at a distance, saw the occurrence and heard a report but observed no lightning.

_A metallic screen appears to protect the interior from the action of a current, as well as from static induction._

Dr. Franklin found he could not destroy a wet rat by artificial electricity, although he could a dry one.

The first lightning conductor was erected in England at Payneshill, by Dr. Watson, in 1762.

The lightning conductor should expose a large surface, and should be united with all the great masses of metal in its vicinity. For stationary elevations the conductor should consist of solid or tubular rods or flat plates of metal. We must consider the _mechanical_ action the lightning may produce on the conductor, as well as any possible heating action. Sir W. Snow Harris mentions that there were no signs of fusion in the fragments of the linked brass rod, at Charles Church, Plymouth, torn to pieces in 1824, or in the small pieces of the conductor at the Hotel des Invalides, at Paris, consisting of a strand of twenty iron wires, and which was smashed in 1839.

He says the benefical effect of _superficial_ conductors appears to depend on the removal of the electrical particles further out of the sphere of each other’s influences.

“Thus we find,” says Sir W. Snow Harris, “in a variety of cases of damage by lightning that the passing charge, in striking on large expanded sheets of metal has become comparatively tranquil, and has been traced no further, whilst in striking on large masses of metal exposing but a small surface, it has assumed an intensely active state.”

He goes on to state that the resistance of the conductor must be kept as low as possible, and as neither the resistance nor the heat developed is increased by rolling the wire out into a flat surface, he argues that “there is, consequently, no disadvantage in giving a lightning rod as much superficial capacity as possible, as regards conducting power, whilst, on the contrary, the diminished intensity attendant on it is very advantageous: this effect of superficial conductors appears to depend on the removal of the electrical particles further out of the sphere of each other’s influence.”

_What quantity of metal is requisite for a lightning rod?_ He concludes from the results of a number of accidents that “a copper rod ¾ inch diameter, or an equal quantity of copper under any other form, would withstand the heating effect of any discharge of lightning which has yet come within the experience of mankind.”

_Practical deductions._—“From the various enquiries contained in the first 123 pages of this book, we arrive at the following deductions:—

“1st. Copper is the best kind of metal for a conductor.

“2nd. The quantity of metal should not be less than that represented by the section of a solid cylinder ½ inch diameter.

“3rd. The metal should be placed under as great an extent of surface as is consistent with strength, and should be perfectly continuous.

“4th. The conductor should involve in its course the principal detached masses of metal in the building.

“5th. It should be placed as close as possible to the walls which are to be defended, and not at a distance from them, and be carried at once directly into the ground.

“6th. It should be attached to the most prominent points of the building, and if the length be very considerable its dimensions should be increased.

“Lastly. In extensive ranges of buildings, all the most prominent parts should have long pointed rods projecting freely into the air, and the greater the range of building the higher they should be.

“In particular cases, in which expense must necessarily be considered, wrought iron tubing may be employed; it should not, however, be less than 2 inches in diameter, and 3/10ths of an inch in thickness.”

Insulating the lightning conductor from the building is quite valueless.

The method of fixing lightning conductors to ships is explained at considerable length.

_Range over which the protecting power of the lightning rod extends._—Great doubts exists as to the answer to this question, since in many cases one portion of a building has been struck while a lightning rod in good condition existed close by.

For example, the powder magazine at Bayonne was 56 feet long, 36 feet wide, covered with thick vaulted masonry and a sloping roof with gable ends, protected by plates of lead; the gutters were also of lead, and there were the usual spouts for discharging the rain. The lightning rod projected about 20 feet above the building, and was attached to the lead of the roof by a metallic socket through which it passed, and which was soldered to one of the lead coverings. Instead of being carried, however, directly into the earth at the foot of the wall, it was turned outward at about 2 feet from the ground, and being bent at right angles, was continued on semi-insulating posts of wood into a trench filled with charcoal, distant 33 feet from the wall.

On the 23rd of February, 1829, the building was struck, the point of the conductor melted, and the leaden plates by which it was attached to the wood posts at the foot of the wall, were more or less torn and perforated by holes. No damage, however, ensued to the building in the course of the conductor. At the south-west corner, a sheet of lead covering the gable end was torn out immediately over a point where two stones of the cornice were united by an iron cramp.

Sir W. Snow Harris considers the possibility of this damage having arisen “from the conductor (in consequence of being continued at so great a distance from the building) not offering a sufficiently easy line of transit for the discharge to the earth,” but he rejects this explanation and concludes that the damage arose from the lightning striking the building in two points.

Again, the Heckingham poorhouse, although armed with eight pointed lightning rods, was struck, in 1787, at a point _m_, 70 feet from the nearest conductor _c_.

The squares at _a_, _b_, _c_, _d_, _e_, _f_, _g_, _h_, indicate chimneys to which lightning conductors were attached. The centre range was 108 feet long, the flanks each about 160 feet long: the details of the lightning conductors are not given. One portion of the lightning discharge struck one of the conductors and was carried off by it without damage to the building, one portion struck the building at the point _m_ and also the shed at _s_, doing some damage, and a third portion struck the ground immediately in front of the building near a gate, G.

The ship _Ætna_ was struck in 1830 by several heavy electrical discharges when at Corfu. These for the most part passed down a chain conductor attached to the mainmast. One of the discharges, however, struck the ship near the bow, and exploded about 12 feet above the forecastle close to the foremast, knocking people down, &c.

The Board-house at Purfleet was a lofty building with a pointed roof, well leaded and connected by lead gutters and pipes with the earth, and with wells 40 feet deep for the purpose of conveying water forced up to a cistern on the roof. It was, therefore, only thought necessary to add an iron spike about 10 feet long to the middle of the highest part of the roof. The building, however, in 1777, was struck and slightly damaged at a point 46 feet from the conductor.

Several other examples illustrating how small an area a lightning rod protects follow.

Sir W. Snow Harris further concludes that experience shows that lightning will not leap from a lightning rod to a piece of insulated or semi-insulated metal near it, although a discharge may take place between the rod and a distant metallic mass in connection with the earth, but not otherwise in connection with the rod.

He lastly considers the question, formerly much debated as to whether a lightning rod attached to a house will attract to the house a discharge that otherwise would not have struck it, and he concludes that there is no foundation for the erroneous impression that the existence of a lightning conductor can ever cause damage.

AN ACCOUNT OF THE CHIMNEY OF THE EDINBURGH GAS WORKS. BY G. BUCHANAN,
C.E., F.R.S.E.

[Proceedings of the Royal Scottish Society of Arts, 1850–51.]

(_Abstracted by G. J. Symons, F.R.S._)

This chimney has a total height of 341½ feet (329 feet above ground), it is circular; at the top the internal diameter is 11 feet 4 inches, and the external 13 feet 10 inches; and at the bottom, internal diameter 20 feet, external 26 feet 3 inches.

Respecting the conductor Faraday was consulted, and replied as follows:—

“The conductor should be of ½ _inch copper rod_, and should rise above the top of the chimney by a quantity equal to the width of the chimney at the top. The lengths of rod should be well joined _metallically_ to each other, and this is perhaps best done by screwing the ends into a copper socket. The connection at the bottom should be good; if there are any pump pipes at hand going into a well they would be useful in that respect. As respects electrical conduction, no advantage is gained by expanding the rod horizontally into a strap or tube—surface does nothing, the solid section is the essential element.[4] There is no occasion for insulation (of the conductor) for this reason. A flash of lightning has an intensity that enables it to break through many hundred yards (perhaps miles) of air, and therefore an insulation of six inches or one foot in length could have no power in preventing its leap to the brickwork, supposing that the conductor were not able to carry it away. Again, six inches or one foot is so little that it is equivalent almost to nothing. A very feeble electricity could break through that barrier, and a flash that could not break through five or ten feet could do no harm to the chimney.

Footnote 4:

The very reverse of what was formerly held by high authorities.—[Note
by Editor of Proc. Roy. Scot. Soc. of Arts.]

“A very great point is to have no insulated masses of metal. If, therefore, hoops are put round the chimney, each should be connected metallically with the conductor, otherwise a flash might strike a hoop at a corner on the opposite side to the conductor, and then on the other side on passing to the conductor, from the nearest part of the hoop there might be an explosion, and the chimney injured there or even broken through. Again, no rods or ties of metal should be wrought into the chimney parallel to its length, and therefore to the conductor, and then be left unconnected with it.”

In answer to some further inquiry, Professor Faraday again wrote:—

“The rod may be close along the brick or stone, it makes no difference. There will be no need of rod on each side of the building, but let the cast-iron hoop and the others you speak of be connected with the rod, and it will be in those places at least, as if there were rods on every side of the chimney.

“¾ rod is no doubt better than ½ inch, and except for expense I like it better. But ½ inch has never yet failed. A rod at Coutt’s brewery has been put up at 1½ inch diameter—but they did not mind expense. The Nelson column in London has ½ inch rod, ¾ is better.

“I do not know of any case of harm from hoop-iron inclosed in the building, but if not in connection with the conductor, I should not like it; even then it might cause harm if the lightning took the end furthest from the conductor.”

The following paragraph states what was done:—

“The electric conductor stands 6 feet above the iron top-plate, ⅝-inch round copper, made fast to stone and brickwork with 7⅞-inch copper holdfasts let 4 inches into the masonry or brickwork, with a head on the inside and an eye on the outside to receive the rod as it was carried up. By these holdfasts an ascent can easily be made to the top by a small tackle suspended to the holdfasts. The conductor is metallically connected to all the ironwork on the stalk—the plate on the top, projecting cope, malleable iron hoops, bolts on the top of stone pedestal, and also the ascending chain. The rod descends into a well about 10 feet from the foundation, and is immersed about 8 feet deep in water, and the end turned up 2 feet in a horizontal direction, and flattened.”

PAPERS _relative to_ SHIPWRECKS BY LIGHTNING, _as prepared by_ SIR SNOW
HARRIS, _and presented by him to the Admiralty_.

(August 5th, 1854. Parliamentary Paper. Fcap. folio).

(_Abstracted by Professor W. E. Ayrton_).

Number of merchant ships destroyed by lightning, loss to the country. Application of lightning conductors to ships in 1820. Mode of applying them. Mechanical difficulties; how overcome. The saving to the Exchequer which has resulted.

Long account of various ships in the Royal Navy not provided with lightning conductors, struck by lightning and damaged. Loss of life and injury that has resulted. Long account of ships provided with lightning conductors, and so preserved.

Sir Snow Harris states that “although his system of lightning conductors ought to guard against all those violent and regular shocks of lightning falling within the ordinary experience of mankind, it is not to be expected that the system could guard against every possible kind of atmospheric electrical discharge, be the circumstances what they may, such as thunderbolts, fire-balls; nor is it expected that it should guard against meteorolites, or against sweeping electrical action mixed up with convulsions of nature; nor can it quiet those minor electrical effects producing electric glow; nor can it always obviate that tremendous concussion and expansion of the atmosphere in cases in which a thunder-cloud discharges its lightning in a dense explosion on the masts, and which may rupture, or mechanically tear to pieces, frangible matter.”

STATISTICS OF BUILDINGS AND SHIPS STRUCK BY LIGHTNING. BY F. DUPREZ,
MEMBER OF THE ACADEMY.

[Académie Royale de Belgique, Extrait du Tome 31 des Mémoires, 5th
December, 1857.]

(_Abstracted by Professor T. Hayter Lewis, F.S.A._)

M. Duprez refers to the Report of a Committee of the Institute of France. (Vide _Comptes rendus_, 1852–6.)

He divides the subject into the following heads:—

1. The frequency with which lightning rods are struck.

2. Their terminal points and the effects of the stroke on them.

3. The conductors and their ground connections.

4. The protective power of the lightning rods.

1. _Concerning the frequency with which lightning conductors are struck by lightning._

The author cites 144 cases of lightning rods having been struck. Of these seventeen were struck two or three times, so that the total number of electric discharges on them was 168, as far as recorded.

But very many cases are not recorded at all, _e.g._, from 1793 to 1813 only two cases were noted. The great number of lightning rods struck would seem at first to support the idea that they attract lightning.

But we must compare the number of rods struck with those fixed, and we find from a communication made in 1777 to the Academy of Berlin, that, even then, a large number were fixed to the most important edifices of N. Italy and England.

The same in 1784 to those in the ports of France and to the ships in the said ports.

In 1794 the fortresses of Russia were ordered to be so protected.

In 1769 there were 166 edifices in Hamburg alone, and 104 in its environs, with conductors.

If the number of conductors were so great in the last century, we must conclude that the number of those struck must be very inconsiderable as compared with those fixed.

In Hamburg, _e.g._, not one rod is recorded as having been struck.

In 1785, Ingen-Housz reports that of all the lightning rods placed by his direction on the Austrian powder magazines and other buildings only one had been struck.

In 1772, Franklin wrote, that during the twenty years in the course of which lightning rods had been fixed in America he knew of five cases only in which these rods had been struck.

Sir W. S. Harris reports in 1854, as the results of twenty-two years’ experience, that the number of vessels struck unprotected by lightning rods, as compared with that of vessels protected by his plan, was as three to two.

The above show that the idea of danger from lightning rods is not well founded.

Besides which it must be remembered that they are frequently placed in the most exposed positions, _e.g._, of the 144 rods struck, seventy-four were on ships, and fifteen others on buildings which had been struck before.

One would think that the number of terminals placed on a building would diminish the chances of their being struck, but it does not seem to be so; _e.g._, twelve buildings in the first list had many terminals communicating with a common conductor or different conductors.

Yet the lightning struck, with explosive effect, one or other of the rods of these buildings.

And in each of two cases the lightning struck at once the three rods fixed to a building.

Of the 144 cases above cited:—

74 were to lightning rods fixed on ships
30 were to lightning rods fixed on towers
9 were to lightning rods fixed on powder magazines
31 were to lightning rods fixed on ordinary buildings.
———
144

In forty-four cases where one of Sir W. S. Harris’ conductors was fixed to each mast of a ship, the mainmast was struck twenty-seven times; the foremast was struck fourteen times; the mizen was struck twice; both the main and foremast twice.

2. _As to the points of the lightning rods struck, and the effect produced on them._

(Sir W. Snow Harris’s system as adopted in the British Royal Navy since 1830 is described. They are formed of bands of copper let into the masts. They have no upper terminals or points, and fifty-five are included in the list already quoted of 144 lightning rods struck.)

Of the eighty-nine cases remaining in the list, only fifty-one are recorded as having their upper terminals ended with points.

Of these, thirty had their points melted to a greater or less extent; six of them were of copper or brass; five were of copper gilt or iron gilt; one was of brass silvered; and four were of platinum. The others are not distinctly described, and the sizes seldom given.

One of brass was 25·4 centimetres (c. 10 inches) long, and 5 millimetres (⅕th inch) diameter at its base, and was melted for ¼th of its length.

One of copper was 24 centimetres (c. 9½ inches) long, and 9 millimetres (c. ⅓rd inch) diameter at base, and was almost all melted.

One of platinum was 8 centimetres (c. 3 inches) long, and 1 centimetre (c. ⅓rd inch) diameter at base. This was melted for a length of 5 or 6 millimetres (c. ⅕th inch.)

It results from the above facts that the points of the lightning rods have been much too slender.

The Institute of France recommends, therefore, for the points 2 centimetres diameter (c. ¼th inch) at base, and only 4 centimetres (c. 1½ inches) high, with an angle of opening of 28 to 30 degrees.

It has been urged, especially in Germany, against the employment of pointed upper terminals that these points are fused by the lightning, this fusion being regarded as dangerous on account of its action on inflammable substances near.

As to this, the author cites three cases of buildings set on fire, though protected by lightning rods. But the precise cause of the fire was not ascertained.

Several observations show that the melted metal trickled down the side of the lightning rod.

At Strasbourg the metal was pressed down on one side, and had bent like wax softened by heat. At other times the lightning disperses the melted metal in all directions. (Examples quoted.)

With these facts before us we cannot altogether deny that some danger may arise from the fusion of the metal at the point of the terminal. But this danger can be much lessened, if not removed, by adopting the size, etc., of the lightning rods recommended by the Institute of France.

Besides fusion, the points sometimes show distinct traces of mechanical action caused by lightning.

The author quotes six examples of this where the points had been curved.

This shows the necessity of strengthening the points of the upper terminals. The curvature arises, probably, from the points being much heated by the lightning, and acted on by the wind.

One case is noted of a point which had the appearance of having been struck violently by a hammer.

Also of one in which the base of a point, where it was screwed to the rest of the upper terminal, was split for a length of 11 millimetres (c. ½ inch).

Also of a platinum point screwed on the upper terminal (copper), and retained by a pin, where the stroke tore away the pin, the point falling intact at the foot of the lightning rod.

3. _Of Conductors of lightning rods struck, and their contact with the ground._

The author refers to forty-one cases of lightning rods struck when not on Harris’s principle.

Of these, 5 were of copper bands soldered together; 5 were of copper wire either as rope or chain; 1 was made of bands of sheet iron; 11 were of bars of iron joined by screws or by solder; 3 had pieces of lead between the parts where they were screwed together; 3 were of simple iron wire, or of rope or chain of iron wire; 3 were of iron joined together by hooks; 12 are described as chains (metal not specified); 1 is described merely as a conductor.

The dimensions of the above are seldom given.

The largest bands reported are 16 centimetres (c. 6¼ inches) in width.

The largest bars reported are 55 centimetres (c. 2¼ inches) in width and 15 centimetres (c. ½ inch) in thickness.

The description of the earth connection is also imperfect.

Of eighty-nine lightning rods described as struck, only twelve are noted as having their ends in running water or wells, and one in damp soil.

Fifteen simply entered the ground, it being noted expressly of six of these that it was dry.

In three cases were the lightning rods were struck the author found that the part at the base and in the damp earth had terminated in a plate of lead, protected above the ground by a wooden enclosure.

Three conductors of ships did not communicate with the sea.

Twenty-three cases are noted of ordinary conductors (not on Sir W. S. Harris’s principle).

The lightning melted, or reduced almost to powder, three.

The first was on a house, and was of copper wire, the diameter not known, ending with a chain of iron buried in the earth.

The second was on a ship’s mainmast, and was of iron wire 6 millimetres (c. ¼ inch), diameter, 46 centimetres (c. 18 inches) long, folded at their extremities, and united by rings.

The third (also to a ship) was a rope of three strands formed in the whole of 60 brass wires, each being one half to two-thirds of a millimetre thick.

The two last conductors had their ends in the sea.

The parts of these conductors, in place of being soldered or screwed together, were joined merely by hooks and rings like a surveyor’s chain. Evidently a bad form as their contact is imperfect.

In three other conductors, whose different parts were screwed together with lead between them, the stroke melted the lead.

This shows the danger of lead from its fusibility, in addition to its less conducting power.

The author gives examples of this, wherein a leaden pipe, 8 centimetres (c. 3¼ inches) external diameter, and 13 millimetres (c. ½ inch) thick, was melted.

He quotes Arago as calling attention to the importance of the form of the bends in conductors, abrupt bends being dangerous.

Two examples are quoted to prove this, the conductors having been broken by the lightning stroke at a sudden bend.

To provide lest the lightning, after having struck the lightning rods, should abandon them for larger masses of metal near them, these masses should be made to communicate with the conductors.

Cases are cited where the lightning quitted the conductor and struck metallic bodies near. Also, in respect of painting conductors, the author quotes a case where part of a bell wire adjoined a lead pipe which communicated with the conductor. Part of the wire was painted in oil colour, the other part not. The latter was melted, the first not, but the paint (though otherwise uninjured) had ceased to adhere to it.

Three examples are cited of danger from conductors ending in watertight tanks.

In one case the stroke broke the conductor.

In another it left the conductor and injured the building.

In the third it merely melted the point of the upper terminal.

Nevertheless, it often happens that the lightning, in spite of imperfect communication with the earth, disperses itself inoffensively.

Out of fifteen cases of lightning rods struck, in which the conductors were simply buried, more or less, in the soil, they carried off the stroke in eleven without the buildings being injured, or any trace being left of it, except that the ground was upheaved where the latter was too dry.

The French Institute, in their report on the protection of the Louvre, considered it necessary to employ, under certain circumstances, a conductor with two branches, the one descending into a subterranean source of water, the other communicating simply with the surface of the earth.

On the other hand, Arago thought that conductors need not enter the ground, but communicate only with a metallic surface lying on the ground.

This view is confirmed by the cases which the author mentions where the surface of the earth being wetted by rain formed a conductor.

Nevertheless, the two branches are desirable, in case one should fail.

Fifty-five conductors on Sir W. S. Harris’s system are recorded as having been struck, but the damage was quite trivial.

Two electrical phenomena are to be noted as sometimes occurring when a lightning rod is struck.

First, when a conductor is formed of metallic plates a peculiar noise is heard like water pouring on a fire.

Second (independently of the form of the conductor), electric sparks are emitted from bodies near. The author cites example at Berne, 1815.

4. _Protective agency of lightning rods._

Out of 168 cases of lightning rods struck (_vide_ page 91) there are only twenty-seven (c. ⅙th) in which the buildings or ships have not been preserved, and of this sixth many of the conductors were imperfect; _e.g._, four terminated in earth which was unusually dry, and two of them were of insufficient size.

Another was formed of pieces having their ends hooked.

Two conductors ended in watertight tanks.

Another was in the form of a surveyor’s chain, the parts not being, consequently, in close contact.

Others were badly jointed, or had imperfect communication with the ground or with the sea.

In two cases the stroke broke the conductor at points where its direction was abruptly changed.

In two other cases the lightning left the conductors struck, and fell upon buildings near without causing damage to those on which the rods were fixed.

In the instance of a lightning rod fixed to the mainmast of the _Jupiter_ (1854), the conductor was made of sixty brass wires, one half to two-thirds of a millimetre (0·02 inch) thick, and was broken by the stroke into thousands of pieces. The Institute Committee concluded that the lightning was not conducted by all the wires of the conductor. Those which it followed were insufficient to transmit it; some were melted, some broken. The Committee recommended, therefore, that each metallic wire be tinned separately at the extremity of the conductor, and soldered thereto for a length of about a decimeter (c. 0·4 inch), so as to form a metallic cylinder.

In the last six cases the particulars of the lightning rods are not given sufficiently to show the cause of their failure, but five are described as being of chain or ropes of metal wire.

It results from the above facts that when the lightning rods have proved insufficient protection, their failure has been owing to defects in their construction; it is rather surprising to find how well buildings and ships have been protected, even when the lightning rods have not been well constructed.

In every one of the fifty-five cases where Sir W. S. Harris’s rods were fixed they have protected the ships, except that not having points some slight damage has sometimes occurred to the tops of the masts.

This shows their superiority over ropes or chains.

Arago thought that lightning rods were protection against ordinary lightning, but not when it assumed the form of fire-balls. The author cites several examples to show that this opinion was not well founded.

He considers a perfectly constructed lightning rod to be a perfect safeguard.

But he adds that the lightning stroke produces electric disturbances in its vicinity, although the building be intact.

He cites an example of this in respect of a prison whose inmates (300) experienced a great enfeebling of their muscular power during some seconds.

Very few records exist relating to the area of action of lightning rods, and the elements for determining their protective power are slight. The author gives a table showing the heights of points, horizontal distances, &c., in certain cases, and cites four instances of ships whose foremasts were struck although the mainmasts had lightning rods, and one where the mizen was struck though the fore and mainmasts were protected.

TABLE GIVEN BY M. DUPREZ.

┌─────────────────────┬───────────────────────┬───────────────────────┐ │ │ IN METRES. │ IN ENGLISH FEET. │ ├─────────────────────┼─────┬─────┬─────┬─────┼─────┬─────┬─────┬─────┤ │ │ 1st │ 2nd │ 3rd │ 4th │ 1st │ 2nd │ 3rd │ 4th │ │ │Case.│Case.│Case.│Case.│Case.│Case.│Case.│Case.│ ├─────────────────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┤ │Length of upper │ 1·5│ 3·4│ 1·5│ 2·3│ 5│ 11│ 5│ 8│ │ terminal, or height│ │ │ │ │ │ │ │ │ │ of point above │ │ │ │ │ │ │ │ │ │ that portion of the│ │ │ │ │ │ │ │ │ │ building on which │ │ │ │ │ │ │ │ │ │ the upper terminal │ │ │ │ │ │ │ │ │ │ was fixed. │ │ │ │ │ │ │ │ │ │Vertical height of │ 1·5│ 7·6│ 6·7│ 71·2│ 5│ 25│ 22│ 232│ │ point above the │ │ │ │ │ │ │ │ │ │ place struck. │ │ │ │ │ │ │ │ │ │Horizontal distance │ 15·2│ 7·3│ 17·4│ 59·9│ 50│ 24│ 57│ 197│ │ of place struck │ │ │ │ │ │ │ │ │ │ from the base of │ │ │ │ │ │ │ │ │ │ upper terminal. │ │ │ │ │ │ │ │ │ └─────────────────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┘

These instances show that we should be misled in considering, as being protected, a circular space whose radius was double the height of the lightning rod.

The protected radius appears to be only equal to double the simple height of the upper terminal above any required point, and reckoned horizontally from a point vertically under the conductor.

[It will be observed that M. Duprez here contradicts himself in two consecutive sentences, and in a subsequent part of his work (p. 30) of the Memoir, he again says: “Aucun des cas indiqués dans le numéro précédent n’infirme la règle généralement admise, savoir que la sphère d’action d’un paratonnerre s’étend, dans toutes les circonstances, à un espace circulaire d’un rayon égal au double de la longeur de la tige, c’est-a-dire de la hauteur de la pointe au-dessus de la partie du bâtiment sur laquelle la tige est fixée.”

But the table given by M. Duprez gives two instances in which the stroke fell within the radius of once the height.—Ed.]

RESUMÉ.

In the paragraphs which the author numbers 1, 2, 3, 4, and 6, he refers to former statements as to the proportion of lightning rods struck, &c. (_Vide_ page 91, &c.)

5. There being several terminals on an edifice does not seem to diminish the chances of each being struck.

7. In vessels, when the three masts have lightning rods, the mainmast is most frequently struck.

8. Refers to Sir W. S. Harris’s lightning rods as being without terminal rods or points.

9. The points of ordinary lightning rods have been made too slight.

10. Out of fifty-one cases of lightning strokes, thirty points have been more or less melted; and the fusion is not without danger to the buildings.

11. The lightning often leaves traces of mechanical action more or less decided.

12. Refers to defective constructions of ordinary lightning rods.

13. Lead plates in conductors composed of bars joined together are dangerous.

14. So are abrupt bends.

15. Conductors should communicate with the masses of metal near.

16. And must not end in watertight tanks. But

17. Conductors often protect buildings, though the ground connections are imperfect.

18. It is well for a conductor to have two branches, viz., one in water, and the other on the surface of the ground.

19, 23. Refers to the complete efficiency of Sir W. S. Harris’s conductors.

20. Mentions the noise, electric sparks, &c., given off during a stroke, as before stated (page 95).

21. Mentions the efficacy of lightning rods generally.

22. Their failure being owing to defective construction.

24. There is no proof that the electricity being in the form of a ball has been the cause of any conductor’s inefficiency.

25. The lightning rarely bursts on a building or ship without striking the lightning rod placed on it. Exceptions have, however, occurred in ten cases, as here described. But

26. None of these instances invalidate the rule generally admitted, that the protective action of the lightning rod extends, under all circumstances, to a circular space whose radius is equal to double the length of the upper terminal, _i.e._, the height of the point above the part of the building on which the upper terminal is fixed.

ON ATMOSPHERIC ELECTRICITY.

BY REUBEN PHILLIPS. LONDON, 1863.

(_Abstracted by W. H. Preece, C.E._)

This is a pamphlet of seven chapters, and fifty-seven pages, written to ventilate the author’s own notions of the nature of electricity and its production in the atmosphere. He considers electricity to be two fluids of a species of substance, consisting of separated subsidiary atoms. “Electricity is comparable to a flying bullet; the _vis vivâ_ of the bullet is like electrical intensity, and the mass of the bullet answers to the quantity of electricity.” What the subsidiary atoms are like he does not say.