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which served to conduct it. This in general prevented verifying Mr. Franklin's hypothesis ; but Mr. Canton was more fortunate, as appears by the following letter from him to Mr. Watson, dat ed from Spital-square, July 21, 1752.
“ I had yesterday, about five in the afternoon, an opportun ty of trying Mr. Franklin's experiment of extracting the electrical fire from the clouds; and succeeded by means of a tin tube, be. tween three and four feet in length, fixed to the top of a glass one, of about 18 inches. To the upper end of the tin tube, which was not so high as a stack of chimneys on the same house, I fastened three needles, with some wire; and to the lower end was soldered a tin cover to keep the rain from the glass tube, which was set upright in a block of wood. I attended this apparatus as soon after the thunder began as possible, but did not fiod it in the least electrified, till between the third and fourth clap; when applying my knuckle to the edge of the cover, I felt and heard an electrical spark; and approaching it a second time, I received the spark at the distance of about half an inch, and saw it distinctly. This I repeated four or five times in the space of a minute, but the sparks grew weaker and weaker, and in less than two minutes the tio tube did not appear to be electrified at all. The rain continued during the thunder, but was considerably abated at the time of making the experiment.”
Mr. Wilson likewise of the Society, to whom we are much obliged for the trouble he has taken in these pursuits, had an opportunity of verifying Mr. Franklin's hypothesis. He informed Mr. W., by a letter from near Chelmsford, in Essex, dated Aug. 12, 1752, that on that day about noon, he perceived several elec. trical snaps, during, or rather at the end of, a thunder-storm, from no other apparatus than an iron curtain rod, one end of which he put into the neck of a glass phial, and held this phial in his hand. To the other end of the iron he fastened three needles with some silk. This phial, supporting the rod, he held in one hand, and drew snaps from the rod with a finger of his other. This experiment was not made on any eminence, but in the garden of a gentleman, at whose house he then was.
Dr. Bevis observed, at Mr. Cave's, at St. John's gate, nearly the same phænomena as Mr. Canton. Trisling as the effects here mentioned are, when compared with those which we have received from Paris and Berlin, they are the only ones that the last summer here has produced ; and as they were made by persons worthy of credit, they tend to establish the authenticity of those transmitted from our correspondents.
Considerations to prevent Lightning from doing Mischief to
great Works, high Buildings, and large Magazines*.
By Mr. Wilson, F.R.S. Long experience, since the discovery by Dr. Franklin, has now established a truth among philosophers, that lightning, like the electric fluid, passes more freely through iron, copper, and other metals, than through dry wood, stone, or marble. Instances of this truth are innumerable : and to be convinced of it, we need only trace the late violent effects of lightning on St. Bride's church, and the houses in Essex.street, &c. For, on examining these buildings, it appears that there are certain thick bars of iron, through which the lightning has passed, without producing any visible effects; and, on the contrary, in certain parts where the junctions of those bars with the stone, or wood, are made, there the lightning, rushing from the iron, has broke the stone to pieces, and shivered the wood. From the like experience we also learo, that if the iron is too slender for conducting the lightning, it is either dashed into pieces, or exploded like gun-powder; just in the same manner as we are able, by the electric power, to break and dissipate in vapour a very slender wire. Bars of metal, of a proper thickness, and conveniently disposed, seem therefore neces. sary for the security of such buildings.
It is to be noted, that the mischiefs caused by lightning are not always owing to its direction from the clouds to the buildings, or other eminences, and thence to the earth; but sometimes, on the contrary, from the earth, buildings, and other eminences, to the clouds. For the principle on which its direction depends, appears to arise from the restoration of a certain equilibrium, in a subtile
* See farther on this subject, the articles in Section v.
and elastic fluid, previously disturbed by various causes. Now, according to the laws of elastic fluids, the endeavour to restore the equilibrium of such a fluid, will be in that direction where the resistance to its passage happens to be the least. On this principle we therefore see a necessity, either to open a passage for it to go freely through, by placing certain bars of metal properly, or to stop the passage of the fluid through such buildings entirely. The last method would be dangerous to put in practice; because, if high buildioys were so secured, the lightning would then attack the lower buildings, which are far more numerous, and probably would destroy a greater number of people, cattle, &c. Whereas, if the first method is preferred, the high buildings will then tend to protect the lower ones more effectually; and may with pro. priety be considered as so many pipes to carry off the lightning quietly, either from the earth to the clouds, or from the clouds to the earth. And that several proper conductors are necessary to carry off the lightning, more readily than some of the accidental or partial conductors in a large town are capable of, appears from this ; that we are able to collect small quantities of the electric fluid, with a slender apparatus in our hands only; whilst it is exposed in the street, garden, or other open place, during the hovering of such clouds as occasion violent lightning.
From repeated observations of this kind, there is reason to believe, that the quantity of lightning at particular times, is so very great, that it would be dangerous to invite it to any buildings, and that unnecessarily, in the most powerful manner we are able; by suffering the several conductors to end in a point at the top. Oo which account it is apprehended, that pointed bars, or rods, of metal, ought always to be avoided. And as the lightning must visit us some way or other, from necessity, to restore the equili. brium, there can be no reason to invite it at all; but, on the con. trary, when it happens to attack our buildings, we ought only so to contrive our apparatus, as to be able to carry the lightning away again by such suitable conductors, properly fixed, as will very little, if at all, promote any increase of its quantity, . To attain which desirable end, in some degree at least, it is proposed, that the several buildings remain as they are at the top; that is, without having any metal above them, either pointed or not, by way of a conductor. On the inside of the highest part of
such building, and within a foot or two of the top, it may be proper to fix a rounded bar of metal, and to continue it down along the side of the wall to any kind of moisture in the ground. But if the building happens to be mounted with an iron spindle, for supporting a vane, or other ornament, and it should not be conve. nient to have it taken away, then the bar of metal ought to com. municate with that spindle. And as to the diameter of such a metal bar, it will probably depend on the height of the building; for it is apprehended the great church of St. Paul's, to complete the partial conductors (which are the metallic cross, ball, gallery, dome, &c.) and secure it effectually, would require a bar of metal two inches diameter, if vot more; and a building like the British Museum, one considerably less. But it appears there is no occa, sion for any at that repository, as it is already provided, though from accident, like many other buildings, with very effectual con. ductors. The copings of the roof, and the several spouts, which are continued from it into the ground, being all of lead.
That conductors ought to be thicker than is generally imagined, seems to appear from a late instance taken notice of in St. Bride's church, by Mr. Delaval, and Dr. Watson, where an iron bar, 2ų inches broad, and half an inch thick, or more, was bent and broken asunder by the violence of the lightning. The Eddystone Lighthouse, which stands on a rock, surrounded by the sea, the work of Mr. Smeaton, was thought to be an object very likely to suffer by lightning; and the more so, as the top of it consisted of a copper ball, two feet in diameter, with a chimney of the same metal, passing through it down to the second floor, but no farther. Directions were therefore given to make a communication of metal from the lowest part of the copper chimney down to the sea; which was executed accordingly about the year 1760, or soon after the building was finished. Now if, instead of the copper ball, a pointed bar of metal had been put in its place, or above it, and communicated with the conducting matter below, there is no saying what might be the consequence of so powerful an invitation, to an edifice thus particularly situated.
Since the former part of this paper was communicated to the R.S., that is, on the 5th of August, 1764, I received the follow. ing account from Captain Dibden, commander of a merchant ship, who says, that in the year 1759, he was taken by the French, and carried prisoner to Fort Royal, in Martinico. That in removing him thence some time after, and on foot, to St. Pierre, which is about twenty miles, his conductors, or guard, stopped at a small chapel, five miles from the last place, to shelter themselves from the heavy rain which fell during a violent thunder-storm.
That the chapel had no steeple or tower belonging to it, but stood on an eminence, with three or four poor low houses near it. That soun after they were thus sheltered, a violent flash of lightning struck two soldiers dead, who had been leaning against the wall of the chapel, between two buttresses, and not far from the rest of the company, being all on the leeward side of the chapel. That it made an opening in the wall about four feet high, and about three feet broad, and in that part only against which they rested.
That Captain Dibden, along with other persons, entered at this hole immediately after, to see if any other damage had been done to the chapel. That they observed a square bar of iron near the hole, and on the ground, about four feet long, and 14 inches thick, making an angle with the wall, as they suppose, to support the upper part of an inclined tombstone, which was also throwo down and broken to pieces. That this bar was joined in the middle to one end of another bar, about one foot long, and one inch thick, which laid horizontally, and, passing to the wall, had been there fastened with lead. That the lightning, in rushing along the in. clined bar, had wasted or reduced its thickness in some places very considerably, insomuch that it looked like a burnt poker which had been long used; and broke the bar into two pieces, about an inch above the joining of the lesser bar, the ends of which had a burot flaky appearance. That the other parts of the bar were changed in colour to a grey, or whitish hue, resembling iron after it has been exposed to a violent heat and then suffered to cool. That the horizontal bar had also undergone an extraordinary change by the lightning, but particularly at that end next the wall of the chapel, it being reduced from one inch in diameter to the size of a slender wire, but tapering towards the wall. That when the soldiers rested against the wall, their heads were about the same height with the shortest bar; and, from what he can recol. lect, were very near being opposite to that end which was inserted in the wall. That the two soldiers were forced from the wall at the same instant by the lightning ; so that their feet, which were