ELECTRICITY

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electricity l-.as opened for farther discoveries, and for the satisfac-tion of that curiosity, which before attended the contemplation ofs© many wonderful phenomena as have been explained by thisscience, there are two great advantages derived from electricity;the one is a defence against the direful effects of lightning, and theother a remedy for many disorders incident to the human body.

4 In order to guard edifices or ships from being damaged by light-ning, it was judiciously proposed by Dr. Franklin, to raise a me-tallic conductor some feet above the highest pait of the building,and continue it down the wall till it penetrated some feet into theground; by this means the house could never receive any da-mage; for whenever the lightning should happen to fall upon it,it is evident that the conductor, being of metal, and higher thanany part of the building, would certainlv attract it, and, by con-ducting it to the ground, hinder that building from receiving anydamage; it being well known that electricity always strikes thenearest and best conductors that it meets within its way. ‘ Thereasonableness and truth of this assertion has been confirmed bynumberless facts, and the practice of raising such conductors hasbeen found exceedingly useful, particularly m hot climates, wherethunder-storms arc very frequent, and the damages occasioned bythe same, too often experienced. In regard to the constructionof such conductors, there have been some controversies amongelectricians; and the most advantageous manner of using themhas not, without a great many experiments, and but very lately,been ascertained. Some philosophers have asserted, that suchconductors should terminate in a blunt end, that they might theless invite the lightning from the clouds; for a blunt end will notattract electricity from so great a distance as a sharp point. Butother philosophers have thought a pointed termination to be muchpreferable to a blunt one; and their assertion seems, on the fol-lowing accounts, founded on much better reasoning. A sharp-pointed conductor, it is true, will attract electricity from a greaterdistance than a blunt one, but at the same time will attract andconduct it by little and little, or rather by a continued stream, inwhich manner a remarkably small conductor is capable of con-ducting a very great quantity of electricity; whereas a blunt ter-minated conductor attracts the electricity in a full separate bodyor explosion, in which manner it is often made red hot, melted,and even exploded in smoke, and by such a quantity of electricityas perhaps would not have at all affected it, if it had been sharplypointed. A sharp-pointed conductor certainly invites the matterof lightning easier than a blunt one; but to invite, receive, andconduct it in small quantities, never endangers the conductor:and the object of fixing a conductor to a house, is to protect thehouse from the effects of, and not the conductor from transmittingthe lightning. It is an observation much in favour of sharp-pointed conductors, that such steeples of churches, and edifices ingem ra), as are terminated by pointed metallic ornaments, have■yerv seldom been known to be struck by lightning; whereas othersthat have fiat or blunt terminations, and have a great quantity ofmetal in a manner insulated on their tops, arc often struck by it,and it is but seldom that they escape without great damage. How-ever, it happened not long ago, that a building furnished withmany sharp conductors was struck by lightning. Besides theseconsiderations, a sharp-pointed conductor, by the same propertyof attracting electricity more than a blunt one, may actually pre-vent a stroke of lightning, to do which a blunt-ended one is abso-lutely incapable. For an edifice of a moderate size, one con-ductor is perhaps sufficient; but, in order to secure a large build-ing from sustaining any damage by lightning, there should be two,three, or more, conductors, in proportion to the extent of thebuilding. In ships a chain has often been used for this purpose,•which, on account of its pliableness, has been found very conve-nient, and easy to be managed among the rigging of the vessel;but as the electricity finds a great obstruction in going throughthe several links, for"which reason chains have been actually bro-ken bv the lightning, so their use has now been almost entirelylaid aside; and, in their stead, copper-wires a little thicker than agoose-quill have been substituted, and found to answer very well.One of these wires should he elevated two or three feet above thehighest mast in the vessel; this should be continued down themast, as far as the deck, where, by bending, it should he adaptedto the surface of such parts, over which it may most convenientlybe placed, and, by continuing it down the side of the vessel, itshould be always made to communicate wiih the water of the sea.

I In regard to personal security, in case a thunder-storm were tohappen while a person is in a house not furnished with a properconductor, it is advisable not to stand near places where there isany metal, as cliimnies, gilt frames, iron casements, or the like;but to go into the middle of a room, and endeavour to stand or situpon the best non-conductor that can be found at hand, as an oldcnair, a stool, &c. If a storm was to happen whilst a person isin the open fields, and far from any building, the best thing he cando is to retire within a small distance of the highest tree or treeslie can get at; he must by no means go quite near them, bntshould stop at about fifteen or twenty feet from theii outermostbrandies; for if the lightning should fall thereabout, it will veryprobably strike the trees; and in case a tree was to be split, he issafe enough at that distance from it.' We shall conclude this sec-tion with a short extract from the earl of Stanhope’s learned work,entitled Principles of Electricity, containing the requisites neces-sary for the proper construction of conductors for lightning, in or-der to preserve buildings from its depredations ; and w hich arequoted with approbation by Mr. Cavallo. These requisites are,“ I. That the rods lie made of such substances as are the best con-ductors of electricity. 2. That the rods be uninterrupted and per-fectly continuous. 3. That they be of a sufficient thickness. 4.That they be perfectly connected with the common stock. 5,That the "upper extremity of the rods be as acutelv pointed as pos-sible. 6. That it be very finely tapered. 7. That it be promi-nent. 8. That each rod be carried in the shortest convenient di-rection, from the point at its upper end to the common slock. 9 ,That there be neither large nor prominent bodies of metal uponthe top of the building proposed to be secured, but such as areconnected with the conductor by some proper metallic communi-cation. 10. That there be a sufficient number of high and point-ed rods: and, 11. That every part of the building be very sub-stantially erected.” He also advises that the upper 15 "or 20inches of the rod should be copper, as iron is subject to rust, andrust is a bad conductor.

Theory of Electricity.

The mind of man is not satisfied with a simple knowledgeof the facts, which an attention to the phenomena of naturepresents to his view'. Fie wishes to account for them; to irac*the causes from theirel'fects; andaithougliphilosophersam! sages havebeen repeatedly, and almost constantly, foiled in their endea-vours to discover first causes, yet the same principle -1111 ex-cites the inquisitive investigator to build theory after theory, andto erect one hypothesis upon the ruins of another. We cannot,therefore, be surprised that the wonderful phenomena of electri-city should have excited this active principle of human nature toaccount for them, upon the first observation of them. In fact,this was even attempted by Thales, who first observed the atlrac-tive power of amber. At this property he was so much surprised,that he reckoned the amber to be animated. With regard to theconjectures of Theophrastus on this subject, we are entirely in thedark; but, among the succeeding electricians, all the phenomenawere derived from unctuous effluvia emitted by the excited elec-tric. These were supposed to fasten upon all bodies in their way,and to carry back with them all that were not too heavy. For, atthat time, effluvia of every kind were supposed to return to thebodies from which they were emitted ; as nobody could otherwiseaccount for the substance not being sensibly wasted by the con-stant emission. When these light bodies, oti which the unctuouseffluvia had fastened, were arrived at the excited electric, a freshemission of the effluvia was supposed to carry them back again.But this effect of the effluvia was noltliought of till electric repulsion,as well as attraction, had been fully observed. '1 he discovery ofa difference between conducting and non-conducting substances,threw considerable difficulties in the wav of those who maintainedthe hypotheses of unctuous effluvia. When (lie Newtonian phi-losophy began to be pretty generally received, the terms attrac-tion and repulsion were quickly introduced into electricity, aswell as other brandies of philosophy ; and the elec trie effluvia, in-stead of being of an unctuous nature, were said to be of an attrac-tive or repulsive one. At the same time, the apparent stop w liichis put to the progress of these effluvia by any electric substance,introduced a question not yet well decided, viz. Whether electricbodies are penetrable by the fluid or not? When M. Du Faydiscovered the two opposite species of electricity, at that time dis-1 tinguished

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