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directed upwards or downwards at pleasure, and the lowei 

 part of the rod terminated upon substances which were 

 imperfect conductors of electricity. On directing the 

 point towards the sky when a thunder-cloud passed over 

 the church, electrical sparks issued in abundance from 

 the foot of the rod ; but when the point of the rod was re- 

 versed so that the bend was upwards, few or none were 

 obtained. The conducting-rod set up by Professor Rich- 

 man, at St. Petersburg, may be said to have been the 

 cause of his death (in 1753) ; but the house would most 

 probably have been struck if there had been no conductor. 

 The immediate cause of the melancholy accident was that 

 the rod led into the apartment, and the unfortunate Pro- 

 fessor was standing too near its lower extremity. (Phil. 

 Tmns., vol. xlviii.) 



Some doubt however still exists concerning the distance 

 to which the protecting influence of the rod extends, but 

 it is generally supposed that this influence is limited by 

 the circumference of a circle described about the rod with 

 a radius equal to double its height above the top of the 

 building. 



When the magazine at Purfleet was struck by lightning 

 in 1777, the shock took place on an iron cramp which 

 united two stones of the cornice, at the distance of 24 feet 

 from the thunder-rod, measured horizontally ; and the 

 upper extremity of the rod was 11 feet above the top of 

 the roof on which it was erected. Again, in 1781, the 

 workhouse at Heckingham in Norfolk, though provided 

 with eight rods, was si nick by lightning at a spot which 

 was distant 5."i feet measured horizontally from the nearest 

 rod, while the pointed summit of the latter was 22 feet 

 above the level of the part struck ; and many other in- 

 stances might be cited in confirmation of the supposition. 

 It may be inferred therefore that large buildings ought to 

 be secured by several rods, and that the less these are 

 elevated the greater ought to be their number ; also that 

 no point of the building ought to be at a greater horizontal 

 distance from the nearest rod than twice the height of the 

 rod above the top of the building. Mr. Cavendish, Dr. 

 Priestley, and other English philosophers, recommend them 

 to stand 10 feet above the roof; but in France their 

 height is sometimes more than 30 feet. 



The most elevated objects are not always those which 

 are struck by lightning ; for in 1829, the sails of a mill at 

 Toothill in Essex being at rest, the electric fluid left un- 

 touched the arm, which was in a vertical position, and fell 

 on a knob of iron at the middle of one of those which wits 

 inclined to the horizon in an angle of 45 ; and it has fre- 

 quently happened that buildings containing in their walls 

 conducting substances have been struck much below their 

 summits by a lateral action of the fluid; bars or plates of 

 metal in the side walls having served to attract it more 

 powerfully than the materials on the roof. This happened 

 to the cathedral at Lausanne in 1783. 



In order that the points of rods may not become blunt 

 by rust, they are generally made of copper and covered 

 with gilding ; but it appears to be the practice of late to 

 make the upper part of the rod of platinum, this metal 

 being not only unaffected by the corrosive power of the 

 air or rain-water, but also incapable of fusion by heat. As 

 the erection of a thunder-rod requires in general an ex- 

 pensive scaffolding, the latter metal, even though more 

 costly than iron or copper, will from its durability be in 

 reality more economical than either of the others. 



Thunder-rods are frequently made to terminate at the 

 upper extremity with one point in a vertical position, and 

 about this a number of points diverge from the rod at dif- 

 ferent inclinations to the horizon : by this construction 

 there is not only a probability that some of the points will 

 be acute when others may have been blunted by the action 

 of the atmosphere, but also among them there will always 

 be one which presents itself in the most favourable posi- 

 tion for attracting the electric fluid. Observations have 

 not yet however been sufficiently multiplied to enable phi- 

 losophers to decide whether conductors so terminated have 

 any advantage over the more simple rods of Franklin. 



The thunder-rod should be thick enough to carry the 

 electric fluid to the ground without being melted by it ; 

 in general a cylindrical rod about half an inch in diameter 

 will be sufficient to prevent this effect from taking pi. 

 whether of iron or copper, it should be covered above 

 ground with a coating of paint ; and the part undur ground 

 u usually formed with two or more branches in order to 



facilitate the passage of the electric fluid into the earth. 

 It must be observed however that, if the lower part of an 

 iron thunder-rod descends into water, it is liable to become 

 corroded by rust ; and a dry soil is not a good conductor 

 of electricity. On the Continent it has been the practice 

 to let the lower part of the rod enter into a pit filled with 

 ashes of burnt wood or powdered coke. If the rod is 

 made of a kind of metal which does not become corroded 

 in water, its lower extremity may be allowed to enter 

 about three feet below the surface : the water should be 

 that of a natural pond, for, if it be in an artificial reservoir, 

 the walls of the latter, being sometimes formed of, or 

 covered with, conducting substances, may prevent the 

 electric fluid from diffusing itself in the earth. 



When a building is provided with several rods, each of 

 these should be continued quite to the ground : at the 

 level of the parapet the several rods should be connected 

 together laterally by slender iron bars ; and the plates of 

 iron which enter into the construction of roofs should in 

 like manner have a metallic communication with each 

 other. As the thunder-rod is necessarily made to follow 

 the outline of a cornice and roof, the part below that 

 which projects above the roof may be made of metallic 

 cords, in order to avoid the formation of angles in its 

 length ; for experience has shown that lightning in de- 

 scending a rod has quitted the latter at its angles, and, 

 after passing through the air, has struck objects which 

 were situated in the line of its first direction. In the ex- 

 pectation of being able to attract the lightning entirely 

 away from powder-magazines, or any building containing 

 explosive materials, thunder-rods have been attached to 

 masts at the distance of a few yards from the building : 

 this practice has many advocates, and the only objection to 

 it is, that it. is attended with considerable expense. 



Many lofty buildings contain in their construction the 

 means of securing them from the effects of lightning, and 

 such is the case with the Monument on Fish-Street Hill ; 

 this building, whose height is more than 200 feet, is 

 crowned with a metallic ball surrounded by bands which 

 terminate in points tending upwards. From the ball 

 descend four bars of iron which serve to support a flight of 

 steps, of the same metal, and one of them is in connection 

 with the iron railing which extends from the balcony to 

 the ground. In 1764 lightning struck the steeple of St. 

 Bride's Church in Fleet Street, and descended from thence 

 along an iron bar about 20 feet in length and two inches 

 in breadth, which was almost buried in the stones : the 

 electric fluid left no traces of its passage along this bar, 

 but at the place where the metal terminated the damage 

 commenced, the stones being destroyed or thrown to a con- 

 siderable distance. A similar accident has this summer 

 (1842) occurred to the church of St. Martin in Westminster. 



A ship at sea, like an edifice on land, may, when there 

 is an accumulation of electric matter in the upper part of 

 the atmosphere, be struck alofl ; or, when the atmosphere 

 is in a contrary state, the lower part of the ship may be 

 struck, the lightning in the latter case ascending along the 

 mast : and ships unfurnished with metallic conductors 

 have frequently suffered serious injury during thunder- 

 storms, while those which have been so provided have ge- 

 nerally escaped. When Captain Cook was at Batavia, 

 lightning fell on the sliip with such force that the shock 

 resembled an earthquake ; the conductor, which was very 

 slender, appeared to be on fire from the mast-head to the 

 sea, yet no damage was done to the ship. In 1814, while 

 a great, storm raged at Plymouth, of all the ships then in 

 port only one was struck by lightning, and this alone was 

 not provided with conductors. And in the roads at Corfu, 

 three violent flashes struck the ship Etna, which had conduc- 

 tors, without doing any injury, while two ships not far dis- 

 tant, being destitute of such protection, were much damaged. 



The rigid bars of Franklin are oonsidered inapplicable, 

 as conductors, to ships, and instead of them chains of 

 copper have been generally employed ; these are attached 

 to the masts at their upper extremities ; and, following the 

 standing rigging, they pass down the ship's sides into the 

 water. Objections have been made to them on account of 

 their want of continuity, and their liability to be injured or 

 broken during storms ; and copper links attached to the 

 masts by flexible spiral wires were suggested by Mr. Singer. 

 M. le Roy also recommended a chain of copper rings 

 which were to encircle the main-topmast backstay ; but 

 neither of these methods has been put in practice. In 



