A TREATISE on ATMOSPHERIC ELECTRICITY.

(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.

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 airexpanded by heat, carrying metallic dust and earthy particles ejected from volcanoes, or carried up byevaporationor other causes, and diffused over an immense surface in the upper regions of the air. “The lightning carries, like aploughshare, the accumulated matter in its progress, and, by the powerfulelectrical 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 stonesextra 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.”

(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. Thenfollows 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.

(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 innatureas indegreefrom 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.

(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 theshortest, but, in addition, thelargestpath.

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.

Ifoutsidethe tower the danger would be greater. He recommends that themetalcross on the steeple bereplacedby astoneone, 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.

(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 themechanicalactionthe 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 ofsuperficialconductors 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 pointm, 70 feet from the nearest conductorc.

View and Plan of Heckingham Poorhouse

The squares ata,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 pointmand also the shed ats, doing some damage, and a third portion struck the ground immediately in front of the building near a gate, G.

The shipÆtnawas 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 ½inch 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 joinedmetallicallyto 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.

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’sbrewery 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.”

(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. (VideComptes 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:—

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 thelightning, 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 tracebeing 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 (videpage91) 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 theJupiter(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.

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.]

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. (Videpage91, &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 (page95).

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.

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