tion from lightning cannot be claimed even in the neighborhood of a good lightning-rod. And a tree may add to the danger, instead of lessening it, for one who stands too near the trunk or under the overhanging branches, since the roots of a tree are not good conductors. I shall now make some remarks on the manner of constructing the lightning-rod, though it would be impossible to exhaust this subject in any narrow limits. First, in respect to the material of the rod, -Which metal is the best? Iron is strong, and can resist mechanical violence, but it rusts, and the oxide formed is a poor conductor. Brass grows brittle, and copper, therefore, though expensive, being durable and a good conductor, is preferred. Secondly, in considering the form of the rod, I do not lay much stress upon the shape of the cross section. The square figure with its edges may have some advantages over the circular section. But I am not certain that as much is not lost by the facility afforded for a lateral discharge at a dangerous point, as is gained by relieving the rod of a part of its charge all along the four edges. These edges are also relied upon to discharge the cloud quietly, as the points at the top of the rod discharge it; and for this purpose the rod is twisted so that its edge may be presented to all points of the horizon. The twisting slightly injures the conducting power of the rod, and cannot be needed for the object in view, as the lightning is not tied down to a geometrical straight line for its orbit. The only important question which has ever been raised concerning the shape of the rod is an old one, and it was soon put to rest. In 1764, Nollet, a rival of Franklin, encouraged the idea that the points which the American discoverer recommended for the top of the lightning-rod provoked the attacks of the clouds. In England, George the Third had parasites about him who flattered the political prejudices of the king, and advocated blunt rods because Franklin insisted on points. Mr. Wilson, a member of the Royal Society, published a paper in opposition to the points, alleging that they invited the electrical fluid in the clouds. Nairne, well versed in electrical experiments, wrote upon Franklin's side. The far-sighted efficiency of the pointed rod in disabling the clouds, while yet a great way off, was proved by Beccaria's experiment on an interrupted rod. The sparks at the break betrayed the passing electricity.* With a break of one eighth or one tenth of an inch a constant succession of sparks will be seen during a storm. Captain Wynne on one occasion found them to continue, at an accidental fracture in a rod, for two hours and a half. The destruction by lightning, in 1769, of the powder-magazine at Brescia, awakened the attention of the British government to the safety of their own magazines, at Purfleet. At the request of the Board of Ordnance, Dr. Franklin visited Purfleet, and recommended the use of pointed lightning-rods, such as had been used with success in America for twenty years. But Mr. Wilson, "then of some note as an electrician," advised the adoption of blunt conductors. On account of this difference of opinion, the Royal Society was consulted, and, in 1772, a committee, consisting of Cavendish, Watson, Franklin, Wilson, and Robertson, men eminent in electrical science, was appointed to suggest the best means for protecting these powder-magazines. The committee adopted Franklin's views in regard to the superiority of pointed over blunt conductors, and the report, drawn up by Franklin himself, was signed by all the members except Mr. Wilson.f The objection of Mr. Wilson to the pointed rod was the same as that of Nollet; namely, that the point invited and increased the lightning. "Every point, as such, I consider as soliciting the lightning, and, by that means, not only contributing to increase the quantity of every actual discharge, but also frequently occasioning a discharge where it might not otherwise have happened." But Franklin refuted this *Annals of Electricity, X. 127, 133, 161, 180. ↑ Sparks's Franklin, I. 342, 430. position of his opponent in the paper which he read to the committee, entitled, 66 Experiments, Observations, and Facts tending to support the Opinion of the Utility of long pointed Rods for securing Buildings from Damage by Strokes of Lightning.' Unfortunately, the magazine at Purfleet, which was provided with pointed rods, according to Franklin's advice, was struck by lightning in 1772, though without suffering any damage. This revived again the controversy between Franklin and Wilson in regard to pointed and blunt conductors, in which the Court sided with Wilson; for it was now 1777, and not 1772, and the battle of Bunker Hill had been fought in the mean time. At the instigation of parties hostile to Franklin and flatterers of the king, the pointed conductors were removed from the queen's palace, and blunt ones substituted in their place. But Franklin's fame was not disturbed thereby, neither was Franklin himself. When he heard of Dr. Ingenhousz's indignation at the change, he said: "He seems as much heated about this one point as the Jansenists and Molinists were about the fire." Franklin then added the following noble sentiments, worthy to be placed by the side of Kepler's enthusiastic challenge to mankind upon the discovery of his three celebrated laws: "I have never entered into any controversy in defence of my philosophical opinions; I leave them to take their chance in the world. If they are right, truth and experience will support them; if wrong, they ought to be refuted and rejected. Disputes are apt to sour one's temper and disturb one's quiet. I have no private interest in the reception of my inventions by the world, having never made nor proposed to make the least profit by any of them. The king's changing his pointed conductors for blunt ones is therefore a matter of small importance to me. IfI had a wish about it, it would be, that he had rejected them altogether as ineffectual; for it is only since he thought himself and family safe from the thunders of Heaven, that he dared to use his own thunder in destroying his innocent subjects." A subscription was raised at court to enable Wilson to make some experiments in the Pantheon favorable to knobs. But Henley, Nairne, and Lord Mahon, men of weight in electrical science, exposed the fallacy of Wilson's arguments. The Privy Council applied to the Royal Society to investigate the subject again. It was referred to a new committee, and this committee indorsed the conclusions of the earlier committee. The Royal Society was urged to change their report, but they steadily rejected any interference with their scientific privileges; the President, Sir John Pringle, declaring that "he could not change the laws of nature." For this loyalty to nature and this quasi disloyalty to the king, he was tormented until he resigned his office. In France the lightning-rod sends up one solitary aspiring point to disenchant the thunder-cloud. But in Germany, England, and America, it is a common practice to surround the principal, vertical point with a cluster of subordinate and inclined points, which stand ready to charge with fixed bayonet upon the hostile electricity of the sky, from whatever quarter it may threaten an attack. The multiple points may also serve to make up by their number for the imperfection of any one; an imperfection which arises from its oxidation by the air or its fusion by lightning. These imperfections from these causes have not been overlooked. The iron point has been gilded, or, better still, a gilded point of copper has been used. In 1790 Robert Patterson of Philadelphia proposed to make the points of plumbago, on account of its ability to resist fusion. But the improvements which Wollasten introduced into the mode of purifying platinum and rendering it malleable, have rendered the great resistance of that metal to the influence of the air or of heat available in the selection of a proper material with which to point the lightning-rod. Too much, however, cannot be said in disparagement of points, however patented, made, † Sparks's Franklin, V. 435. † I. 343. not of platinum, but of a platinum needle sunk into another metal so soft that it can be melted down in the flame of a candle. The plan, adopted by some, of pointing a rod with a magnetized needle, rests upon no scientific basis whatever. The question is often asked, whether electricity exhibits signs of inertia, or shows any tendency to leave a circuitous path and dash off in a tangent. The common impression with scientific and practical men is, that electricity moves without any perceptible inertia. Hence in the construction of lightning-rods no care has been taken to avoid short turns and sharp angles in the longitudinal shape of the conductor. But the attention of both cannot fail to be arrested by the facts to the contrary cited by Arago in his posthumous work, "Le Tonnerre." * The efficiency of a lightning-rod depends upon its height above surrounding objects. This is proved by experiment. Several rods of unequal height are placed near one another, and it is observed that the highest carries down the largest amount of electricity, this amount being measured for each rod by the number of sparks which can be counted in a given time at a break made for that purpose in the rod. An experiment with artificial electricity would be equally instructive. Cæteris paribus, the most elevated object will be chosen as the principal conductor by the lightning. Therefore, the rod must rise higher than the objects which it is designed to pro tect. It is of much importance to know the necessary height of a rod above these objects, or, in other words, to know the horizontal area which is protected by a rod of a given height. Franklin did not give attention to this inquiry. In England rods rose ten feet above the roof, in France they mounted sometimes to thirty feet. In 1788, J. B. Leroy, "guided by vague analogies," gave the rule that the space protected was a circle of sixteen metres in radius, when the height of the rod was five metres above the building. In 1823, the Physical Section of the French Institute was consulted on this subject by the Minister of War, and adopted as its own the opinion of Charles, that a lightning-rod protects, at its point of contact with the top of the building, a circular space around, the radius of which is double the height of the rod above that point. This rule has been generally adopted since that time, though it is not known upon what grounds Charles established it. In extending this rule to different levels above and below the point of contact of the rod with the highest point of the building, we might suppose that so large a circle could not possibly be protected on the higher levels, while a still larger one might be protected on the lower levels. And thus we might reach the generalization, that the whole space protected from the top of the rod down to the ground would be included in a cone the radius of whose base was twice its altitude. By referring to the case of a tree struck at Cambridge, as described by Dr. Winthrop, and other examples, Arago has concluded that bodies are not exempt from danger within this cone; while there is no instance to overthrow the supposition of a protected cylinder of space having the uniform radius which Charles's law would give it at the top. Pouillet, in the sixth edition of his Elemens de Physique,† adopts a rule, nearly agreeing with that of Charles, as a deduction from experiment. We shall avoid the necessity of raising the rod to an inconvenient height above the roof of the building, if we use several rods, each of which will protect its own charmed circle, while the united conducting power of all will be in requisition to carry off extraordinary discharges, at whichever rod they may first strike. In oblique discharges, which come from clouds when they are not vertically above the point struck, the degree of exposure is measured, not by the vertical elevation of a rod, but its oblique distance; and it may therefore happen that the highest point will escape, while one at a less distance takes the discharge *Euvres de François Arago, I. 368. + II. 771. from the clouds. In 1824, Leslie* advised that advantage should be taken of the copper gutters and spouts of buildings to help the lightningrod to carry off its electrical burden; and Mr. Henry has made a similar suggestion in regard to the tin roofs of houses, if they are connected with the ground by metallic pipes. The precautions recommended in the protection of powder-magazines from lightning are peculiar. These magazines are generally surrounded by an atmosphere of fine powder-dust, ready to be inflamed by a small spark originating in some accidental want of continuity in a lightning-conductor. Hence, as early as 1776, Toaldo advised that conductors should never be placed directly upon these magazines, but upon masts at the distance of about ten feet from them. Where a mistake might involve so great destruction, Toaldo thought it wise for men to stand upon the defensive, and not to be too familiar with the tremendous energies of nature. Voltaire has likewise said: "There are great dignitaries whom it is only safe to approach with great care; and lightning belongs to the same class." When Gay-Lussac made his report, in the name of the Commission appointed at the request of the Minister of the Interior, by the Physical Section of the French Academy, to draw up instructions as to the best method of preparing lightning-conductors, he adopted with approbation the old suggestion of Toaldo in relation to powder-magazines. But if, as has been intimated before, the proper interpretation of Charles's laws requires that the radius of the space protected should be measured by the height of the rod, not above the ground, but above the highest point of the object to be protected, the erection of substantial masts to sustain these high rods, and at distances from each other, all around the magazine, not exceeding one quarter of the height† of the rod itself, will involve no inconsiderable expense. Sturgeon has proposed to line the walls of powder-magazines with metal, which would protect the interior of the building from any inductive action. Did not experience prove the contrary, it would seem superfluous to say that a good lightning-rod must be uninterrupted throughout its whole length; and, when it reaches the ground, must be bent away from the foundations of the building it is intended to protect, and enter to such a depth into the sub-soil as to be surrounded by ground always moist. The lower end of the rod may be soldered to a large plate of metal, or it may be surrounded by a large body of charcoal; not common charcoal, but such as has been heated red-hot, or by coke. Sometimes the rod can communicate with a well or other reservoir of water. It has been proposed that, where practicable, lightning-rods should be attached at the bottom to the waterpipes under ground. It is an old saying, that the danger is over when the lightning reaches the well (ocean). An artificial fountain may not be large enough to make a good discharging train. Great improvements have been made, since the time of Franklin, in the manner of jointing the several pieces of which the lightning-rod is composed. Formerly they hooked upon each other like the links of a chain. Dr. King of Boston turned up a point on one piece of the rod at right angles to the length. This point entered an eye upon the next piece of rod, and so on. These points answered for oblique charges, but nothing in this arrangement prevents two pieces of rod from disconnecting, if the attachment to the side of the building should give way. Mr. Orcutt connects the rods by means of a hollow nut cut on the inside, to which is affixed a point. The two pieces of rod screw into this nut until they touch one another. Mr. Strong overlaps the two pieces of rod, and screws a pointed piece through both. But this enumeration does not exhaust all the varieties. *New Phil. Journ., 1824, p. 38. ↑ Peschel's Physics. The consequences of being struck by lightning at sea are so fearful and often so fatal, that the smallest chance of such an accident ought to be guarded against at any cost. Sir William Snow Harris of England deserves credit for having called public attention to this subject of late, and for having improved upon the old method of protecting ships from lightning. That method was simply this: a chain, generally stowed away, and left to be raised to the masthead in the hurry of preparations to meet a storm. Sometimes the men employed in raising it have been killed by the lightning before their work was finished. Sometimes the chain was carelessly short, so that during the rolling of the ship the lower end was lifted out of the water. Besides, in a chain there must be great obstruction and heat at the links. It is no wonder that accidents from lightning still happened to ships so poorly provided, and that these accidents brought the chain itself into discredit; so that frequently even public vessels were not provided with one. In the English navy the old chains were of copper, one sixth of an inch in diameter, and the links two feet long. They were only supplied when demanded. In the French navy metallic ropes were used. Harris's lightning-conductor for ships consists of flat strips of copper let into the masts. They contain twenty times as much metal as the old English chains. There is no break in the strips of copper, as Roberts objected. They do not interfere with the rigging, and they are always in position ready for use. The mast helps to conduct, as is seen by experimenting on a fine wire completely insulated and on another laid upon a piece of wood. The strips of copper connect with the copper sheathing, and bands also lead off under deck to the knees and other pieces of iron in the sides of the vessel. Vessels having Harris's conductors have escaped, when others in company with them have been struck. It has been objected to Harris's plan, first, that it weakened the spars, and secondly, that it injured their pliability. But good testimony has been adduced to refute these charges. Mr. Sturgeon, the well-known English electrician, has urged the expense; which amounts, for labor and material, to 1585 dollars. Mr. Sturgeon also finds fault because the mizzen-conductor, in one case at least (H. M. S. Java), passed through the powder-magazine; and objects that the plan is not adapted to oblique attacks of lightning. Finally, Lieutenant Green avers that the method of Harris is an ill-copied contrivance of Marrot, published in the Naval Chronicle in 1812. Harris replies, that the article to which reference is made does not mention the name of Marrot, but that of Le Roy, and that nothing is said in it of conductors fixed in masts. Leslie, in 1824, spoke of ribbons of copper extending from mast to keel.* Lieutenant Green asks in what respect Harris's plan differs from Singer's, who, nine years before, introduced bolts through the keel of the vessel; or from that in use in the French navy thirty years before. Mr. Roberts objects to Harris's method of conducting electricity, that he supposes the surface to act exclusively. Mr. Harris denies the charge, but Roberts quotes a passage from him in which he says that a smaller quantity of metal formed into a hollow cylinder would be better than a solid one on account of the interior surface. Among the other plans which have been contrived for protecting ships from lightning I may allude to Sturgeon's, who recommends wire-ropes, to be attached to the sides of shrouds. This arrangement, he argues, would be 500 dollars cheaper than that of Harris. Murray applied his hollow tubing to the marine, by having sliding joints in it, as in the telescope, so that when the topmast was taken down the lightning-conductor might be proportionately shortened. Mr. Roberts, with a view to remedy the defects of the old chain conductor, namely, that when the royal and top Edin. Phil. Trans., 1822, p. 38. |