Popular Science Monthly/Volume 25/September 1884/Protection Against Lightning I
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Protection Against Lightning I
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THE first lightning-conductor was erected by Benjamin Franklin upon his own house in Philadelphia in 1752. The invention is, therefore, now a little more than one hundred and thirty years old. Franklin was led to the investigations which resulted in its construction by the fortuitous circumstance that, about six years previously, he had been present at a lecture on electricity delivered in Boston by Dr. Spence. In the same year—that is, in 1746—he received a present from Peter Collinson, a member of the Royal Society in London, who was also the agent of the Library Company in Philadelphia, of one of the London electric tubes, and an account of some experiments that had recently been made by Dr. Watson, Martin Folkes, Lord Charles Cavendish, Dr. Bevis, and others of their contemporaries. The idea had already suggested itself to these investigators that the luminous gleam which was elicited from glass tubes when they were rubbed in dark cellars, in performance of the frequently repeated and fashionable experiment of the day, might possibly be of a kindred nature to the lightning of the thunder-storm. In a book describing some "physico-mechanical experiments" that he had made, published in London in 1709, Francis Hawksbee remarked that the luminous flash and crackling sounds produced by rubbing amber were similar to lightning and thunder. In 1720 Stephen Gray, the pensioner of the Charterhouse, so celebrated for his electrical investigations, boldly and uncompromisingly affirmed that, "if great things might be compared with small," the light and sound called forth when glass rods were rubbed were of the same nature as lightning and thunder. Franklin, from the time when the electrical experiments came under his notice, enthusiastically adopted this view. In a letter written to a friend in 1749, he very clearly expressed his reasons for this belief. In this communication he insisted upon the facts that the electric spark gives light like lightning; that the luminous discharge follows a similar crooked track; that this discharge is swift in its motion, is conducted by metals, is accompanied by an explosion when it escapes, rends bodies that it passes through, destroys animal life, melts metals, sets fire to inflammable substances, and causes a smell of sulphur—all of which attributes seemed to him to point to the identity of the phenomena. He also observed that the electric discharge was attracted by points, and stated that he was bent upon ascertaining whether lightning had not the same tendency. In the autumn of the following year he wrote to Mr. Collinson to say that he had satisfied himself in this particular; that he was entirely convinced of the identity of the so called electricity with lightning; that he believed the damage done by lightning descending from the clouds to the earth might be altogether prevented by placing iron rods, with sharp points, upon the summits of buildings; that he intended to test experimentally the soundness of his belief in that matter; and that he hoped other persons would assist him in his labors by following his example. This was virtually the definite forecast of the conductor which Franklin attached to his house in 1752.
In the mean time the suggestion that buildings might be protected from lightning by the use of iron rods with sharp points was incidentally communicated by Mr. Collinson to the editor of the "Gentleman's Magazine" in London, who, at once perceiving the practical importance of the hint, offered to print an account of Franklin's views in the form of a pamphlet. This offer was accepted, and, in the month of May, 1751, a pamphlet was published in London, entitled "New Experiments and Observations on Electricity made at Philadelphia, in America, by Benjamin Franklin." The pamphlet was not very warmly received in England, but it was enthusiastically welcomed and appreciated in France. Count de Buffon had it translated into French, and the translation appeared in Paris within four months of the publication of the original pamphlet in England. It was soon afterward translated into German, Italian, and Latin. The attention of scientific men in Paris was quickly drawn to the method of defense proposed by Franklin, and M. Dalibard, a man of some wealth, undertook to erect the apparatus at his country residence at Marly-la-Ville, some eighteen miles from Paris. The situation of the house was considered to be eminently favorable for the purpose, as the building stood some four hundred feet above the sea. A lofty wooden scaffold, supporting an iron rod an inch in diameter and eighty feet long, was erected in the garden. The rod was finished at the top by a sharp point of bronzed steel, and it terminated at the bottom, five feet above the ground, in a smaller horizontal rod, which ran to a table in a kind of sentry-box, furnished with electrical apparatus. On May 10th, when M. Dalibard was himself absent in Paris, the apparatus having been left temporarily in the charge of an old dragoon named Coiffier, a violent storm drifted over the place, and the old dragoon, who was duly instructed for the emergency, went into the sentry-box and presented a metal key, partly covered with silk, to the termination of the rod, and saw a stream of fire burst forth between the rod and the key. The old man sent for the Prior of Marly, who dwelt close by, to witness and confirm his observation, and then started on horseback to Paris, to carry to his master the news of what had occurred. Three days afterward, that is, on May 13, 1752, M. Dalibard communicated his own account of the incident to a meeting of the Académie des Sciences, and announced that Franklin's views of the identity of the fire of the storm-cloud with that of the electrical spark had been thus definitely established.
Before the success of M. Dalibard's experiment could be reported in America, however, Franklin had secured his own proof of the identity by the memorable experiment with the kite, so well known to the scientific world. He was anxiously waiting for the erection of the first steeple in Philadelphia for the opportunity which this would afford him for the support of a lofty iron rod, when the happy idea occurred to him to try, in the mean time, upon some suitable occasion, whether he could not contrive to hold up a lightning-conductor toward a storm-cloud by means of a kite. On the evening of July 4th, that is, fifty-two days after the experiment of M. Dalibard, his kite was raised during a thunder-storm, and, with the help of his son, he drew electric sparks from the rain-saturated string, as the two stood in the shelter of an old cow-shed in the outskirts of Philadelphia. He held the kite by a silken cord that was attached to a key at the bottom of the string, and with this arrangement he charged and discharged an ordinary Leyden-jar several times in succession. Franklin at first not unnaturally conceived that he had actually drawn the lightning down from the storm-cloud. He was, however, no doubt mistaken in this. The storm-cloud had inductively excited the neighboring surface of the earth, and what Franklin saw was the electric stream escaping out through the wet string toward the storm-cloud to relieve the tension set up by this induction. It was in the summer of the same year, after the performance of this world-renowned experiment with the kite, that Franklin attached to his house a lightning-conductor, which was composed of an iron rod, having a sharp steel point projecting seven or eight feet above the roof, and with its lower end plunged about five feet into the ground.
As a matter of course, the new doctrine of Franklin and his allies was not received without considerable opposition. A sharp shock of an earthquake having been experienced in Massachusetts in 1755, this was forthwith attributed to the evil influences of Franklin's lightning-rods. A Boston clergyman preached against them in 1770 as "impious contrivances to prevent the execution of the wrath of Heaven." Even as late as 1826, an engineer in the employment of the British Government recommended that all lightning-rods should be removed from public buildings as dangerous expedients, and in 1838 the Governor-General and Council of the East India Company ordered that all lightning-rods should be removed from public buildings, arsenals, and powder-magazines throughout India, and only became reconciled to their restoration after a large magazine and corning-house, not furnished with a conductor, had been blown up during a storm.
Franklin was so much in earnest in reference to his invention that he sent a friend at his own charge through the principal towns of the New England Colonies to make known the powers and virtues of the lightning-rod. In the "Poor Richard" for 1758, a kind of almanac or manual which he was at that time publishing, he gave specific instructions for the erection of his rods. The second conductor which he himself constructed was placed upon the house of Mr. West, a wealthy merchant of Philadelphia. A few months after this had been erected a storm burst over the town, and a flash of lightning was seen to strike the point of the conductor, and to spread itself out as a sheet of flame at its base. It was afterward found that about two inches and a half of the brass point had been dissipated into the air, and that immediately beneath the metal was melted into the form of an irregular blunt cap. The house, nevertheless, was quite uninjured. The sheet of flame seen at the base of the conductor Franklin correctly ascribed to the ground having been very dry, and to there not having been a sufficiently capacious earth contact under those circumstances. He nevertheless shrewdly, and quite justifiably, assumed that in this case Nature had itself pronounced an unmistakable verdict in favor of his invention.
The controversy concerning the efficacy of lightning-rods continued to agitate the councils of scientific men, notwithstanding this memorable demonstration of their efficiency; but, upon the whole, the new doctrines made their way into the confidence of the intelligent classes of the community. The most important circumstance in connection with the early fortunes of the invention, perhaps, was the admirable series of reports and instructions which were issued by the French Government between the years 1823 and 1867, and to which Mr. Anderson now once again, and not superfluously, draws public attention in his recent pamphlet entitled "Information about Lightning-Conductors issued by the Academy of Sciences of France." The first of these reports was drawn up in 1823 by Gay-Lussac, the discoverer of the law of the expansibility of gases, the companion of Humboldt, and the distinguished meteorologist who first ascended four miles and a half into the air in a balloon. The second and the third were prepared in 1854, and in 1867, by M. Pouillet, the director of the Conservatoire des Arts et Métiers in Paris, and the author of a well-known work on the elements of experimental physics and meteorology, which has been translated into many languages. These reports, although drawn up by an individual, were the results of the deliberations and experiments of a considerable number of scientific men, acting as a commission, and comprising among them such distinguished names as those of Poisson, Fresnel, Becquerel, Duhamel, Fizeau, and Regnault. In the first of these reports, that, namely, of Gay-Lussac, which was adopted by the Academy of Sciences on April 23, 1823, it was premised as a kind of axiom that there are no bodies which do not offer some resistance to the transmission of electricity, and that conductors of small diameter offer more resistance than those which are of the same composition and of larger size. The electrical state was conceived in these investigations as consisting of some kind of matter—as depending upon molecules which are mutually repulsive, and which therefore tend to separate and disperse themselves through space, and which are only retained upon the surface of solid bodies by the pressure of the atmosphere. When the electric matter escapes, it seeks the earth under its tendency to diffuse itself over the most capacious conductors it can find, selecting the most perfect of them that are within its reach, but dividing itself in proportion to their individual capacities of accommodation, when several conductors of unequal power are open to its transmission. A storm-cloud, hovering above in the air, attracts toward the nearer part of the terrestrial surface an electrical matter of a contrary nature to its own, and drives back into the ground an electrical matter of the same nature as its own. Each prominent part of the ground is therefore, for the time, in a state of electrical tension during the presence of a neighboring storm-cloud, and becomes a center of attraction toward which the lightning inclines. When the prominent object is in good connection with the ground, its electrical matter may shoot forth toward that of the cloud, and make a path between it and the cloud. If the prominent body projects as a sharp point toward the cloud, the escape of the electric matter from it to the cloud becomes very rapid, and the lightning strikes to it from the cloud, from a greater distance. It was further conceived that a good conductor protected from any violent discharge a circular space whose radius was twice the height of the rod. An iron bar three quarters of an inch square was taken to be of sufficient dimensions for the construction of a conductor, because no instance had been known of a rod a little in excess of half an inch in diameter having ever been fused or raised to a red heat by lightning. Even small rods or wires that were dispersed by the passage of lightning had served to convey it to the ground, and had protected surrounding objects from single strokes. Trees were recognized as dangerous to animals taking shelter near their trunks, because they do not convey a lightning-discharge with sufficient rapidity to the ground, and because they are worse conductors themselves than animal bodies. But the discharge will not in any case leave a good conductor, well connected with the ground, to strike a living animal placed near its course. The terminal rod of a conductor was ordered to be two and a half inches square at its base, and to taper to a height of twenty or thirty feet above the building, with a needle of platinum, or of copper and silver alloy, at its top. The base of the rod was to be plunged into the ground, and then led away from the building for fifteen feet, being finally turned down into a hole or well fifteen feet deep, and then divided into root-like ramifications, the whole being well packed round with charcoal to protect the metal from rust. In a dry soil the earth contact was to be twice the length of the one which was deemed sufficient in a wet one. It was above all things insisted upon that too great precautions could not be taken to give the lightning a ready passage into the ground, as it was chiefly upon the freedom of this passage that the efficacy of the conductor must depend. A conductor with insufficient earth contact was stigmatized as being not only inefficacious, but dangerous, because it would attract the lightning without being able to convey it to the ground.
It was further asserted in this most comprehensive and notable report that an experience of fifty years had proved buildings to be effectually protected when good conductors were placed on them. In the United States a number of conductors had been known to have been struck, but in not more than two of these cases had the buildings themselves suffered any damage. It was generally assumed, from the data then at command, that buildings which were protected by lightning-rods were not more likely to have the discharge brought down in their neighborhood on account of the presence of the rods, and it was also held that, even if they were open to such a liability, this could be of no practicable moment, because the power of a conductor to attract the lightning more frequently would, of necessity, also involve the capacity to convey it more freely to the ground. Points were spoken of as undoubtedly tending to neutralize the tension of a charged cloud. Dr. Rittenhouse was referred to as having observed in Philadelphia that the points of lightning-conductors were frequently blunted by fusion without the houses to which they were attached having been in any way injured.
The views advocated in this early code of instructions have been dwelt upon it some detail, in order that it may be seen how effectively this document laid down the broad principles of defense which are acted upon even at the present day. This instruction, after it had been stamped with the approval of the Academy of Sciences, became a sort of popular manual under the weight of this sanction. The Government gave force to the instruction by providing that it should have effect in reference to all public buildings and churches. The report also became the chief authority on the subject in most foreign lands. It likewise served the useful purpose of weakening the opposition, which still endeavored to maintain that disastrous explosions were caused by conductors, and furnished clear and precise rules for construction that were intelligible to ordinary workmen.
In the year 1854 iron was much more generally used in buildings than it had been at an earlier date, and some additional knowledge of the conditions and laws of electrical action had been acquired. The Academy, on this account, thought it well to request the Section of Physics to reconsider the lightning-rod instruction of 1823. This led to the first report, which was prepared by M. Pouillet, adopted by the Academy of Sciences on March 5, 1855, and immediately afterward issued by the Government as an additional instruction. In this document it was held that the large masses of iron employed in buildings certainly serve to attract the lightning. If two buildings of an equal size were similarly placed, the one being exclusively of stone and wood, and the other having large masses of metal in its construction, the lightning would certainly strike the latter and avoid the former, just as, when a ball of metal and a ball of wood are presented together toward a charged prime conductor of an electrical machine, it is always the former, and never the latter, which receives the spark. A dry soil, it was pointed out, does not attract the lightning. But, if, under such a soil, there occur at some depth large masses of metal, or accumulations of water, the lightning would explode through the dry earth, splitting it up as a coat of varnish is pierced by an electric spark. The line of lightning-discharge is always marked out for it beforehand, in conformity with the law of electric tension, beginning at the same instant at both the extremities of the track. The objects which are most liable to strokes of lightning are good conductors that project farthest over toward the clouds.
In the report of 1855 the occasion was used to draw attention to some instructive instances of the mechanical effects of lightning-discharges which had taken place upon the open sea. In 1827 the packet-boat New York, not at the time carrying a conductor, was struck during its passage across the ocean, and a leaden pipe, three inches in diameter and one inch thick, was fused where the discharge escaped into the sea. A chain of iron wire, one quarter of an inch in diameter and one hundred and thirty feet long, having been then hoisted up on one of the masts and trailed in the sea, was struck by a second discharge, and scattered into molten molecules and broken fragments, the bridge being set on fire, although at the time covered by a sheet of hail and a deluge of rain. The Jupiter, in the North Sea fleet, in 1854, carrying a chain of several strands of fortieth-of-an-inch brass wire, two hundred and sixty feet long, hung from the mainmast-head, and trailing seven feet into the sea, was struck, and had the chain scattered into thousands of fragments, without any damage being done to the vessel itself. A Turkish ship cruising near at the time, with a chain from the masthead which did not reach into the sea, had a hole like that which would have been made by a cannon-shot pierced through the hull near the water-line. The inference was drawn from these cases that chains, and especially small chains, were not trustworthy for the purpose of conducting discharges of lightning. The mechanical violence sustained was perceived to be due to the circumstance that the conductors provided were of a bad principle of construction. They were at the least from nine to ten times too small. Conductors provided by engineering art are intended to be struck, but struck in such a manner as to govern the lightning, and to render the heaviest strokes harmless. No case had been known of a continuous iron rod, three quarters of an inch in diameter, or with a sectional area of one and a quarter square inch, having been structurally injured. The cases alluded to were held to demonstrate that conductors must have a sufficient size and thickness of metal, and must be continuous and without defect from end to end. It was definitely settled that, in accordance with these requirements, a square iron rod used as a defense against lightning should have, at least, a diameter of nine sixteenths of an inch, and that a round rod should have a diameter of ten sixteenths of an inch.
Some modification was also made in this instruction in reference to air-terminals. It was considered that a blunt point, fashioned like the apex of a cone subtending an angle of thirty degrees, would be less liable to fusion than a sharper and more attenuated point, and that therefore it should be adopted for the upper terminal, although it might, perhaps, not exert altogether so satisfactory a neutralizing influence. The area protected by a conductor was now considered not to be so definite and certain as it was previously held to be. It was recognized that it would be less in the case of a building with a metal roof, for instance, than in other circumstances. The earth contact, it was remarked, could not be looked upon as efficacious unless it were made, through the instrumentality of sheets of water, at least as large as the area of the storm-cloud, and access to such sheets must be secured by boring both in the direction of the surface moisture and in that of the deeper soil. Chains of red copper with a square section of three eighths of an inch, and weighing a pound and three quarters per yard, were recommended for ships. Such were the principal suggestions of a practical kind that were submitted in this report. In all other particulars the provisions of the earlier instructions were substantially approved and confirmed. There was, however, one incidental remark contained in this excellent report which is deserving of the highest commendation and approval on account of its practical wisdom. This emphasized the necessity for continued and minute observation and study of the effects of thunder-storms, with a view alike to ascertain what it is that lightning spares, as well as what it strikes. It is of the utmost importance, for the advance of man's knowledge in this branch of physical investigation, that all instances of injury from lightning should be immediately examined and tested, and that all facts ascertained should be accurately described and placed upon record.
In the year 1866 the Minister of War in France became doubtful in regard to the measures which were then taken to secure powder-magazines against accident from lightning, and in consequence once again brought the matter formally under the consideration of the Academy of Sciences. It was this action of the minister which led to the third report, also drawn up by M. Pouillet, adopted by the Academy in the beginning of 1867, and shortly afterward issued under the authority of the French Government. In this report the best method of making joints in a conductor by overlapping, riveting, and soldering the contiguous ends, was pointed out, and it was urged that the underground continuation of the rod should be carried on to an adequately moist place, even if miles had to be traversed for the purpose. The increase in the number of air-terminals and the connecting them together were deemed of more consequence than the increasing the height of a smaller number. Secondary terminals were advised for every additional length of thirty-three yards of roof. The expansion of rods by heat was provided for by inserting free semicircular bands of red copper at suitable intervals, four inches of addition to the length being allowed for in every hundred yards of rod.
The example set by France in the preparation of these reports was followed for the first time in England by the appointment of a Naval Commission in 1839 to inquire into the protection of the vessels of the Royal Navy. This commission was formed in consequence of the public attention which had been drawn to the matter by Snow Harris, who stated that, within the forty years that ended in 1832, two hundred and fifty vessels had been more or less seriously injured by lightning. The commission somewhat haltingly reported that there was no harm in lightning-conductors, and that it thought the system of protection might be tried. Snow Harris thereupon introduced the plan of nailing a double set of overlapping strips of copper along the masts. After the adoption of this method the conductors were struck by lightning in several instances, but in no case did the vessels suffer any damage. This excellent system was only superseded in the end by the natural result of the introduction of iron vessels, which made the ships themselves efficient conductors in virtue of the principle of their construction. The original idea of Snow Harris was, indeed, to bring the general structure requiring defense as nearly as possible into the same non-resisting state that it would have if entirely composed of metal. He was knighted for his services in 1847, and in 1855 was employed to design the protection of the then new Houses of Parliament at Westminster, which he carried out by a modification of the plan that he had matured for the protection of the vessels of the Royal Navy. Two-inch tubes of copper, connected by solid screw plugs and coupling pieces, were affixed to all the more elevated portions of the building. The sum of £2,314 provided for the execution of this work was memorable as being the first grant made by the English Parliament for the protection of a public building against lightning.
About ten years after the erection of the lightning-conductors upon the Houses of Parliament at Westminster, it was found to be desirable to provide a similar protection for the magnificent old Hôtel de Ville at Brussels, in consequence of some damage having occurred to the principal tower of the building during a thunder storm. The communal administration of the city had recourse to the Académie Royale des Sciences for advice in the emergency, and a commission, consisting of M. Duprez, M. Liagre, and Professor Melsens, was appointed to give a careful consideration to the matter. Professor Melsens visited Plymouth and London, to consult with Sir W. Snow Harris, and to examine the plan of defense which had been adopted for the Houses of Parliament. Shortly afterward the commission at Brussels submitted to the communal administration the famous plan of lightning-defense which has since been carried out at the Hôtel de Ville, and which has been described in the minutest detail in an illustrated work entitled "Description détaillée des Paratonnerers établis sur l'Hôtel de Ville de Bruxelles," and printed in 1865, in explanation of his views, by Professor Melsens himself.
Professor Melsens's method of defense differs in one important particular from the measures which had been recommended in the Paris instructions, and which have been most generally adopted in England. He had for some time been inclined to advocate the use of numerous rods of small size, rather than one dominant rod of more ample dimensions, whenever large buildings with numerous projecting pinnacles and gables were concerned. His view virtually is that the aim in such cases should be to throw a sort of metallic net broadcast over the building, with salient points carried up into the air at all projecting parts of the structure, and with numerous rootlets plunging down into the conducting mass of the earth beneath; and he contrived an experiment which he was in the habit of exhibiting to his visitors at the laboratory in l'École de Médecine Vétérinaire de l'État, which certainly went very far to justify the position he had taken up. He prepared a spherical case or cage of stout iron wire, and, having inclosed a small bird in this cage, he passed electric shocks through it from a battery of fifteen very large Leyden-jars, without causing either injury or inconvenience to the bird. A couple of little feathered pensioners were maintained at the laboratory for the performance of this experiment, and were subjected to the ordeal a considerable number of times, and there is no doubt could be subjected to it for any number of times, without the remotest chance that they would ever be touched by the terrific discharges that were flashed through the walls of their prison-cell in such close propinquity to them. What happened in the case of the birds in this experiment assuredly would happen also in the case of any building that was encaged in metallic rods in a similar way. No demonstration of a mere physical fact could possibly be more absolute or more complete.
The Hôtel de Ville at Brussels is a large mediæval building, inclosing in its center an open quadrangular court, and surmounted in the middle of its principal face by an elaborately pinnacled tower, 297 feet high, with a gilt statue of St. Michael at the top, standing upon a prostrate dragon and flourishing a drawn sword above his head. There are four galleries on the spire beneath the statue, and there are also six spire-crowned subordinate turrets, and three parapeted gables projecting above the roof from other parts of the building. The statue of the saint is reared upon a lead-covered cupola or platform, and Professor Melsens determined that the point of its sword should serve as the culminant point of his system of lightning-rods; but he also took the precaution of very largely re-enforcing this highest terminal by surrounding the base of the lead-covered platform at the feet of the statue with a chevaux-de-frise of outwardly and upwardly branching rods, constituting a radiant circle of tufted points or aigrettes. There were altogether forty-eight of these points projecting round the feet of the statue to a distance of eight feet in all directions. From these radiating aigrettes, and from the statue standing above, a series of eight iron rods were carried down along the face of the tower and the slope of the roof, through an entire length of 310 feet, to the interior court-yard. But as these rods descended along the perpendicular face of the building they were joined by other similar rods from the various subordinate turrets, pinnacles, gables, and ridges, which all had their own systems of terminal points rising up toward the sky. There were altogether 426 points projecting up from the building. An observer looking down from one of the elevated galleries of the spire took in at a glance quite a little forest of spikes bristling up into the air, which were all in direct metallic contact with the main stems of the conductors.
An even more ample provision was made for the connection of this system of conductors with the ground. The vertical rods were first collected into an iron box fixed about a yard above the ground in the inner court, and filled with molten zinc so as to unite the whole into one continuous block of metal. From the hollow of this box twenty-four iron rods, two fifths of an inch in diameter, issued, and of these a third part was carried to an iron cylinder sunk in a well, another third was connected with the iron water-mains of the town, and the remaining third was put into communication in a similar way with the gas-mains. Professor Melsens estimated that the earth contact which was established by this threefold distribution amounted altogether to 333,000 square yards of conducting communication. Iron rods were used in preference to copper in this construction on account of the cost which would have been entailed if copper had been employed for so extensive a work, and also because Professor Melsens had satisfied himself that iron has more tenacity and power of cohesion than copper when exposed to the disintegrating strain of powerful discharges of electricity. He devised a very pretty experimental proof of this, in which the discharge of a large battery of Leyden-jars was passed through a fine wire of equal dimensions throughout, but of which one half was composed of copper and the other half of iron. The iron portion was converted into a beaded, but still unbroken, strand by the discharge, but the copper part was scattered into a black impalpable powder. It is scarcely too much to say that the Hôtel de Ville at Brussels at the present day, with its lofty aigrette-defended tower, its forest of points, its net-work of rods, and its widely ramifying earthroots, is, as far as danger from lightning is concerned, one of the best protected buildings in the world. It may safely be affirmed that it is quite as hard for the lightning to get mischievously at this building, as it is for the discharge of the Leyden battery to get at Professor Melsens's birds when they are inclosed in their iron cage. In the heaviest of storms Professor Melsens travels about within the meshes of his system of conductors, to investigate their behavior, with the most perfect sang-froid and confidence. In 1866 Professor Melsens examined with great care the transmitting capacity of his system of conductors at the Hôtel de Ville, and in this final investigation he employed all the various means that are now at the command of science. He used continuous currents, instantaneous discharges, sparks from the electrical machine, from powerful batteries, and from a large Ruhmkorff coil, and with all he found that the conductibility of his system was practically perfect.
One of the grounds upon which Professor Melsens adopts his system of multiple rods is the circumstance that an electrical discharge diffuses itself through all the branches of a multifold conductor in proportion to the resistance which is offered by each part, and that it does not all concentrate itself into the shortest and most open path. He has devised some very ingenious experiments for proving this position, and has been able to show the sixty-thousandth part of a discharge passing by a very narrow and roundabout path, when a broad and direct one was open, and traversed by the larger proportions of the discharge. He brought this part of his subject under the notice of the Academy of Sciences of Belgium in a special note, which was printed in their "Bulletins" in 1875.—Edinburgh Review.
- It is, perhaps, worthy of remark that, in this lecture, the experiments were made by the primitive instrumentality of a glass rod and silk pocket-handkerchief.
- M. de Fonvielle says of this plan of defense that Professor Melsens does not leave the lightning a gap that it can get through.