« ZurückWeiter »
receives it from it; and the electrified air, being conveyed upward by various means, communicates its electricity to the clouds.
It has since been observed by Lord Mahon, now Earl Stanbope, that damage may be done by lightning, not only by the main stroke and lateral explosion, but also by what his lordship calls THE RETURNING STROKE; by which is meant, as we shall notice more fully in a subsequent article, the sudden and violent return of that part of the natural share of electricity gradually expelled from some body or bodies within the range of the main stroke, by the additional passage of the electrical atmosphere discharged from the thunder cloud,
(Phil. Trans. Rozier. Payne. Editor.
Invention and curious properties of the Electrical Kite*, 1. Letter from Benjamin Franklin, Esq. to Mr. Peter Collinson, F.R.S. dated
Philadelphia, Oct. 1, 1752. As frequent mention is made in the public papers from Europe, of the success of the Philadelphia experiment, for drawing the electric fire from clouds, by means of pointed rods of iron erected on high buildings, &c. it may be agreeable to the curious to be informed, that the same experiment has succeeded in Philadelphia, though made in a different and more easy manner, which any one may try, as follows:
Make a small cross, of two light strips of cedar, the arms so long as to reach to the four corners of a large thin silk handker. chief, when extended; tie the corners of the handkerchief to the extremities of the cross, so you have the body of a kite, which being properly accommodated with a tail, loop, and string, will rise in the air like those made of paper; but this, being of silk, is fitter to bear the wet and wind of a thunder-gust without tear. ing. To the top of the upright stick of the cross is to be fixed a very sharp-pointed wire, rising a foot or more above the wood. To the end of the twine, next the hand, is to be tied a silk rib. band; and where the twine and silk join, a key may be fastened.
* See further on this subject, section y, l.
The kite is to be raised when a thunder gust appears to be corr. ing on, and the person who holds the string must stand within a door, or window, or under some cover, so that the silk ribband may not be wet; and care must be taken that the twine does not touch the frame of the door or window. As soon as any of the thunder-clouds come over the kite, the pointed wire will draw the electric fire from them; and the kite, with all the twine, will be electrified; and the loose filaments of the twine will stand out every way, and be attracted by an approaching finger.
When the rain has wet the kite and twine, so that it can con. duct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle. At this key the phial may be charged; and from electric fire thus obtained spirits may be kindled, and all the other electrical experiments be per. formed, which are usually done by the help of a rubbed glass globe or tube, and thus the sameness of the electric matter with that of lightning completely demonstrated.
[ Phil. Trans, 1759.
2. The same subject continued.
By W. Watson, Esq. F.R.S. AFTER the communications received from several correspondents in different parts of the continent, acquainting us with the success of their experiments last summer, in endeavouring to extract the electricity from the atmosphere during a thunder-storm, in conse. quence of Mr. Franklin's hypothesis, it may be thought extraor. dinary that no accounts have been yet laid before the Society of our success here from the same experiments. That no want of attention therefore may be attributed to those here, who have been hitherto conversant in these inquiries, he states, that though several members of the Royal Society, as well as himself, did, on the first advices from France, prepare and set up the necessary apparatus for this purpose, they were defeated in their expectations, by the uncommon coolness and dampness of the air here, during the whole summer. They had at London only one thunder-storm; viz. on July 20; and then the thunder was accompanied with rain, so that by wetting the apparatus the electricity was dissipated too soon to be perceived on touching those parts of the apparatus
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 find 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 tin 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. Trilling 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 and elastic finid, 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 buildings 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.
* See farther on this subject, the articles in Section v.
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 bighest part of