Philosophical Magazine Series 5

ISSN: 1941-5982 (Print) 1941-5990 (Online) Journal homepage: http://www.tandfonline.com/loi/tphm16

XLIV. On lightning and magazines

Charles Tomlinson F.R.S. F.C.S.

To cite this article: Charles Tomlinson F.R.S. F.C.S. (1889) XLIV. On lightning and gunpowder magazines , Philosophical Magazine Series 5, 28:174, 368-375, DOI: 10.1080/14786448908619877

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XLIV. On Lightning and Gunpowder Magazines. B~/ CHARLES TOMLINSON, F.R.S., F.C.S., ~'e.~ OME time about the year 1845 I visited the Tower of S London in company with a relative who was fond of architecture, and wished to inspect the remarkable Norman Chapel of St. John in the White Tower. We were informed that the Chapel was closed, on account of a great bulk of the Public Records being stored in it. The authorities~ however, allowed us to view it from an end gallery, which was partly choked up with dusty bundles of paper. We were informed that th.e vaults below the chapel were used as a gunpowder magazine. At this time the Duke of Wellington was High Constable of the Tower. When some one asked whether he was aware that some of the most valuable historical records of the kingdom were placed over the gunpowder magazine, the Duke replied, "I don't see what harm the Records can do to the gunpowder" ! In other words~ he was keeper of the gunpowder but not of the Records. Nevertheless the danger implied by the question was by no means illusory. In 1767 the Church of St./~azaire, at Brescia in Lombardy, was s~ruck by lightning, and a quantity of gunpowder belonging to the Republic of Venice, stored in the vaults of the church and estimated at 207,600 lbs. in weight, exploded, when not only the church but a considerable portion of the town was destroyed, and about three thousand persons werekilled. When the account of this disaster reached England our Government became alarmed as to the safety of our gun- powder magazines at Purfleet, which were qui~e unprotected against the attacks of lightning, and the sought the advice of the Royal Society as to the best means of protection. The Council appointed a committee, consisting of Dr. Franklin, the Hen. H. Cavendish, Messrs. Watson, Robertson, and Wilson, who inspected the magazines, and found them to consist of five buildings, side by side, each abou~ 150 feet long and 52 feet wide, built of brick and

Downloaded by [University of California, San Diego] at 15:49 22 June 2016 arched under the roof, and standing on a chalk foundation. The Report, which was drawn up by Franklin, recommended an efficient system of pointed metallic conductors for each magazine, terminating at the lower extremity in a well of water to be dug at the end of each magazine. All the mem- bers of the committee signed the report except Wilson, who objected to pointed conductors on the ground that they drew the lightning to the building ; and he recommended blunt or Communicated by the Author. On Lightning and Gunpowder Magazines. 369 knobbed conductors as effectually answering the purpose of "conveying away the lightning safely, without that tendency to increase or invite it," that belongs to points. Wilson pub- lished a long defence of his view of the matter, and was ably answered by ~airne and Henley. Franklin admitted that "points drew electricity at a greater distance in the gradual silent way ; but knobs will draw at the greatest distance a stroke." Wilson found many supporters of his views, and the time of the Society was long occupied with discussions as to the respective merits of sharp and blunt conductors. Some even adopted the idea, which has survived even to our own time, that lightning-rods attracted the lightning to the building they were intended to protect, and thus occasioned loss instead of protection. This idea was started by the Abbd Nollet, who was for some time an electrical authority in France. In his Mdrnoire sur les effets du Tonnerre he expressed his opinion in the following terms :--" Toutes ces pointes de fer qu'on dresse en Fair ..... sont plus propres h nous attirer le feu du ton- nerre qu a nous en preserver. The Board of Ordnance, however, resolved to adopt the Report of the Committee, supported as it was by such power- ful names, and accordingly proceeded to erect pointed con- ductors on the powder-magazines. The Report is dated 21st August, 1772. }Cow it so happened that on the 12th May, 1777, the Board-house at Purfleet was struck by lightning at a point upwards of 40 feet from the conductor. The damage was but slight, consisting chiefly of a few stones fastened by iron cramps being thrown down. A similar accident had occurred on the 17th June, 1774, when the chimney of a house at Tenterden was struck by lightning although another chimney 50 feet distant was furnished with a conductor. Such cases as these were made the most of by Wilson in favour of his knobbed conductors, although the whole contest between the sharps and the blunts involved a sort of fallacy, since, as Snow Harris puts it, " any termination which can con- veniently be given to a conductor, even if it were a ball Downloaded by [University of California, San Diego] at 15:49 22 June 2016 a foot in diameter, would be in relation to, say, a thousand acres of cloud, virtually, a pointed conductor." The mistake made by both parties in this controversy was in supposing that a lightning-conductor attracted the lightning ; whereas it is as passive as the rain-pipes destined to carry off the rain. Should a building or a ship form part of the line of least re- sistance between a thunder-cloud and the earth or the sea, the building or the ship will be struck ; but if they are furnished with properly arranged conductors, the lightning will pass Phil. Mag. S. 5. Vol. 28. ~o. 174. Nov. 1889. 2 E 370 Mr. C. Tomlinson on Lightning down without doing any damage, just as the rain passes down the water-pipe. By ~ properly arranged" is meant a conduc- tor so fitted up that at whatever part of the building or ship the lightning may happen to strike, it shall be conducted safely to the earth or the sea. The French at one time had a theory that a lightning.rod protected a surrounding space equal to a circle the radius of which has twice the length of the light- ning-rod. But in the presence of such cases as the above, and others that might be cited, the radius of protection theory has been abandoned by competent observers. But to return to our narrative. The continued opposition of Wilson and his party embarrassed the Board of Ordnance, and its members again applied to the Royal Society for advice. After considerable debate, a second Committee was appointed, consisting of the President and Secretaries together with Messrs. Henley, Lane, and NMrne. They examined the powder- magazines, and again reported in favour of pointed conductors. Whereupon Wilson contrived a number of experiments in support of his views, and forwarded a detailed account of them to the Board of Ordnance, which referred it back to the Royal Society. Several meetings were occupied in the reading and discussion of Wilson's paper, and at length another Committee was appointed, consisting of the I-Ion. H. Cavendish, Henley, Lane, Lord Mahon, Nairne, Dr. Priestley, and Dr. Watson, with instructions to test Wilson's experiments and conclusions, and report the result to the Society. The report was altogether unfavourable to Wilson, who became wroth at this further testimony in favour of Franklin's pointed conductors, and he endeavoured to persuade tile Government that the Society as a body did not agree with the reports of its several committees. Whereupon the Board of Ordnance applied to the President to ascertain whether this were so. The reply was to the effect that the voice of the Society was usually expressed by its Committees, but that in the present case " the Society had no reason to be dissatisfied with the Report of its Committee." England was at this rime at war with her American Colonies, and as the American rebel Franklin was the inventor of pointed Downloaded by [University of California, San Diego] at 15:49 22 June 2016 conductors, Wilson and his friends seized the opportunity of making the quarrel a political one. It is scarcely credibleat the present day that at the time here referred to I those who advocated sharp conductors were to be regarded as rebels and republicans, while those who advocated blunt conductors were naturally good and loyal subjects. Franklin was at this time in France, supporting the claims of the colonists in that country. In a letter to Dr. Ingenhousz, dated 14th October, 1777, after referring to the report which he drew up in 1772, a~M Gunpowder M-agazines. 371 and which expressed his own view and that of the other mem- bers of' the Committee as to the efficacy of pointed conductors~ he adds :q " I have no private interest in the reception of my inventions by the world, having never made, nor proposed to mak% tile least profit by any of them. The King's changing his pointed conductors for blunt ones is therefore a matter of snmll importance to me. If I had a wish about it, it would be that he rejected them altogether as ineffectual. For it is only since he thought himself and thmily safe from the thunder of heaven, that he dared to use his own thunder in destroying his innocent subjects." Meanwhile Wilson was supported by all so-called loyal per- sons~ who took part in the King's resentment against his re- volted subjects. The King, as hinted in Franklin's lette U is said to have ordered blunt conductors to be affixed to his palace, and even endeavoured to make the Royal Society rescind its resolutions in favour of pointed conductors. His Majesty is also said to have endeavoured personally to persuade Sir John Pringle to use his influence in support of Wilson, but that the President had properly replied that he could not re- verse the laws and operations of ~ature. To this the King said," Perhaps, Sir John, you had better resign." It has often been asserted that the suggestion thus put into the King's mouth was never uttered. Dr. Kippis~ F.R.S., Pringle's friend and biographer, has been frequently cited in proof' of the denial. In reviewing a book of mine in the ' Athe- nmum,' the late ProF. De Morgan adopted this view, and we exchanged friendly letters on the subject. The fact is that Dr. Kippis uses very cautious language. He published in 1783 six discourses by Pringle delivered at the Royal Society~ pre- faced by a biographical sketch. After alluding to the annoy- anee felt by his friend at the quarrel over the lightning-con- ductors~ he adds (p. lvii):--" Of this matter the present writer of his life can assert nothing from personal knowledge; for though he was then in the habit of a strict intimacy with Sir John Pringle~ he never heard from him any suggestion of the Downloaded by [University of California, San Diego] at 15:49 22 June 2016 kind that has been mentioned." It must be borne in mind that Prlngle was " Physician to their Majesties/' and that Dr. Kippis was a leading Unitarian divine, whose pen was largely employed on the ~Annual Register,' the Bioyrap]da Britannica, and similar publications~ and that the position of Dissenters, and especially of Unita- rians at the tim% was by no means sat% as we learn from the writings of Locke at an earlier, and from the treat- ment of Franklin and Priestley at a later period. But wimt 2E2 372 Mr. C. Tomlinson on Lightning we know of the King's conduct in this matter is so much in harmony with his character, that the truth of the anecdote is at least probable, seeing that the President apparently acted on the suggos.tion of the King~ and resigned at the next anniversary meeting of the Society, while the Fellows passed a strong resolution in his favour. From a scientific point of view he was certainly better qualified for the office of Presi- dent than his courtly successor~ Sir Joseph Banks*. Pringle died in 1782, and Condorcet delivered his Eloge, which was based on the Memoir by Dr. Kippis. I have ventured to revive this interesting episode in the scientific history of the lightning-conductor from the fact tha~ even now it is a matter of discussion as to the best method of protecting various structures, especially powder-magazines~ from the effects of lightning. The scientific journals and the newspapers fl'e~uently contain accounts of what are called the "vagaries~" and " eccentricities," and "caprices" of lightning, just as if the whole subject consisted of a series of unsolved problems. Writers describe what they have seen and then ask for explanations. Trees are struck and shattered, and the bark and splint~;rs scattered in all directions, and yet the reporter in each case fancies that he sees something un- usual. A recent writer in 'Nature,' in cndeavouring to account for such effects~ has magnified the nitric-acid forma- tion of the Cavendish experiment into the production of nitro-glycerine. In other words, if a flash of lightning passing through the air produces the synthetic result 57205, is it not also capable of producing the compound known by the complex formula C~Hs(N08)a ? Cuvier, in his .Eloge on Banks, says :--"The works which this man leaves behind him occupy a few pages only: their importance is not greatly superior to their extent; and yet his name will shine out with lustre in the history of the Sciences." That is, during upwards of forty years that he presided over the Royal Society, he promoted science by his wealth, rank~ and influence. Cu~;ier's .Eloge was translated by lne in a work entitled ' Sir Joseph Banks and the Royal Society,' published by Parker in 1844. Banks long exercised considerable influence over the election of Fellows. He was accustomed to regard two sets of men as Downloaded by [University of California, San Diego] at 15:49 22 June 2016 eligible for the Fellowship. In the first rank he placed working men of Science; in the second men of rank and wealth, who were likely to patronise Science. When Dr. Vau~'han, a fashionable physician, wished to enter the Society, the President opposed him on the ground that he was not a working man of Science. But some years later when he inhe- rited a fortune and became Sir Hem'y Halford, Banks consented to his admission as a patron of Science. Many of the older Fellows adopted ~he same view ; and I heard it asserted on the occasion when the choice of President rested between the Duke of Sussex and Sir John Herschel, that the former should be preferred on account of his influence with the Govex~ament in furthering the interests of the Society. and GunpowderMagazines. 373 The following case, which has been reported to me fi'om Dresden, and has not, so far as I know, been noticed in any English journal, has produced the usual bewilderment. On the 15th May last the valley of the Upper Elbe was assailed by a violent storm which about midnight burst upon the town and fortress of KSnigstein. Flash followed flash, stroke followed stroke, tile thunder was incessant, and rain fell in torrents. Suddenly towards one o'clock .~.~. a thunder-clap louder than any before was heard, and it was found that the powder-magazine A had been struck by lightning and its contents exploded. The magazine contained~ in addition to stores of gunpowder, upwards of a thousand grenades, shrap- nel shells, and also cartridges. The roof of the magazine was carried off, and the rest oi the structure blown up into the air, while beams and stones were hurled about tar and near, together with fragments of exploding shells. A sentry-box, with a man in it, was whirled away : the man was injured in the head and arm, and lamed. The watchman also and another man were hurt. It is stated that a trifurcated stroke fell ; one prong on the tower of the ibrtress, a second on the Law Court and Casino, and the third on the powder-magazine. The shells continued to explode during many hours~ so that passengers had to be warned off. Among the neighbouring buildings a new canteen was destroyed, the roof of the stables was knocked into holes, the windows of the dwelling-houses colnpletely smashed, both in the ibrtress and in the town. The open spaces within the fortress were covered with wreck, and fragments of cartridges were hanging on the trees all around. Indeed the effects of the explosion could be traced for miles. Persons at a distance, feeling their houses shaken, doors slammed, domestic utensils rattled, animals uneasy, and dogs barking, supposed that an earthquake had taken place. The question now naturally arises, whether this powder-mag- azine was protected by means of properly arranged conductors. The authorities state that the magazine had three lightning-conductors, which had been examined only a few days before the accident, and were reported to he in good order. Downloaded by [University of California, San Diego] at 15:49 22 June 2016 The theory is that the roof together with the conductors was first carried off, and that the lightning then fired the powder. But in the account furnished to me, it is stated as a fortunate circumstance that there was very little wind at the time. If so, how could the lightning carry off the roof, together with the conductors ? It is more reasonable to suppose that the conductors terminated in dry ground and so offered resistance to the lightning, so as to allow it to develop its tremendous heating and properties. 1~o inspection of the ruins 374 Mr. C. Tomlinson on Lightning after such an explosion would throw much, if any, light upon the cause of the accident. As to the inspection of the conduc- tors shortly before the explosion, it is necessary to see the report, if any were made, and also to know whether the inspectors were competent to undertake the task. In the case of the Purfleet magazines, Franklin's arrangement of the protecting conductors was judicious, but the magazines themselves were faulty in construction. The five magazines were placed side by side with interspaces of about 57 feet. If an accident had happened to any one, the others would probably have shared its fate. The barrels of gunpowder were piled on each other up to the spring of the arches, and each barrel was bound with four copper hoops, while a number of iron bars passed through the arches to support the timbers on which a crane was worked, thus forming broken conductors within the building. These iron bars were, however, removed at the suggestion of some Fellows of the Royal Society, with the approval of the Committee. Snow Harris admitted the difficulty of providing complete protection to powder-magazines, from the fact that they are frequently eonstructedin a series of long straggling buildings, and hence are more likely to be struck at some point distant from the conductors. He recommended as the safest form for a powder-magazine a circular building with a conical metallic roof, furnished with a projecting pointed rod from the vertex, the cylindrical body being furnished with several vertical metal rods, attached to the roof and passing into damp ground or water. If such buildings were covered on the outside entirely with metal~ still greater security would be attained. The late Professor Clerk-Maxwell was so kind as to com- municate to me his ideas on the same subject. Under his system the projecting point and the ground connexion were dispensed with, the function of the point being rather to tap the thunder-cloud, or the atmospheric charge, than protect the building ; whereas the object to be attained should rather be to prevent the possibility of a discharge taking place within a Downloaded by [University of California, San Diego] at 15:49 22 June 2016 certain space, such as a gunpowder manufactory or magazine. His system is based upon the postulate that an electric dis- charge cannot take place between two bodies unless the differ- ence of their potentials be sufficiently great, compared with the distance between them. If we can keep the potentials of all bodies within a certain space equal~ or nearly so, no dis- charge will take place between them. We may secure this by connecting all these bodies by means of good metallic con- and Gunpowder Magazines. 375 ductors. But it is not necessary to do so; for it may be shown experimentally that if every part of the surface sur- rounding a certain space is at the same potential, every point within that space must be at the same potential, provided no charged body be placed within that space. It would there- fore be sufficient to surround a powder-magazine with a good conductor, to sheath its roof~ walls, and ground-floor with thick sheet copper, and under such conditions no electrical effect could occur within it on account of any thunderstorm outside. In such case there would be no need of any earth- connexion; a layer of asphalt might be placed between the floor and the ground so as to insulate the building. Should the magazine be struck by.lightning it would remain charged for some time ; and a person standing on the ground outside and touching the wall might receive a shock, but no electrical effec~ would be perceived inside even on the most delicate electrometer*. For ordinary buildings it is sufficient to enclose the build- ing with a network, say, of copper wire, such as No. 4 British wire-gauge (0"238 inch diameter), carried round the founda- tion of the house, up each of the corners and gables, and along the ridges. The copper wire may be built into the wall to prevent theft; and it should be connected to any outside metal, such as the lead or zinc on the roof, and to metal rain- water pipes, &c. Highgate, N., August 1889.

* On this subject see Faraday, 'Expel~mental Researches,' ยงยง 1173-4. An insulated cubical chamber w-as con-structed, coated on the outside with metal, and charged with electricity from a powerful machine. "I put a delicate gold-leaf electrometer within the cube, and then charged the whole by an outside communication very strongly for some time together; but neither during the charge nor after the discharge did the electromeHr or the air within 'show the least signs of electricity." Going further back~ we may refer to Priestley's ' History of Electricity ' (4to, 1775) for some experiments by Franklin and the author. A pair of pith-balls suspended within an insulated tin quart vessel were not in the Downloaded by [University of California, San Diego] at 15:49 22 June 2016 Ieast affected when a charge was given to the vessel (pp. 688 et seq.). It was also found that a wire net could be substitute5 for the tin quart. Hence a wire netting would protect a gunpowder magazine as effectually as plates of metal, but would not be so durable.