pointed metallic substances. These things being observed, place a few pointed wires in the prime conductor of the machine; put the machine in action, and after the lapse of a few minutes the air of the room will become so impregnated with electricity as to be very perceptible to the sense of smelling, and will readily affect a delicate electrometer, particularly if brought into the vicinity of the machine. The odor perceived on this occasion very much resembles that of oxygen gas, or of the atmospheric air in a very clear and frosty night.

387. The charge of a jar is retained in the electric. This is proved in a very satisfactory manner by the following experiments, which show that the coating of the Leyden jar has not quite so much to do with the charge as is generally supposed. The first is given by Mr. Walker in his System of Familiar Philosophy, and is in substance as follows:-Lay a plate of tin or brass on your hand, and on it a plate of glass (rather larger than the metallic plate); on the glass lay another metallic plate, and let this communicate with the prime conductor: thus the glass may be charged. By the edge of the glass disengage it from the two plates, and place two other plates in the same situation, upon and under the glass. If now one knob of the discharging-rod be made to touch the under plate, and the other knob the upper plate, a discharge will ensue the same as if the first plates had remained in their place.

388. The same principle is tnus illustrated by the Leyden jar; we give the process as directed by Mr. Singer :-Procure a jar with a double set of moveable tin-coatings, either of which may be adapted to it at pleasure; the outer coating being a tin can large enough to admit the jar easily within it; and the inner coating a similar can sufficiently small to pass readily in the inside of the jar. The charging wire of the inner coating should be surrounded by a glass tube covered with sealing-wax, to serve as an insulating handle, by which the inner coating may be lifted from the jar when that is charged, without communicating a shock to the operator. Arrange the jar with its coatings, and charge it; it will act in every respect as an ordinary coated jar. Charge the jar, and, without discharging it, remove the inner coating by its insulating handle; if this coating, when removed, be examined, it will be found not at all, or but slightly electrified lift the jar carefully from within its outer coating, and examine that; it will evince no signs of electricity.

389. Let the jar be now fitted up with the other pair of moveable coatings; apply the discharger, and an explosion and spark will follow, which clearly proves that the accumulation is retained by the attractive power of the glass, and that the coatings are only useful as conductors to the charge.

390. Velocity of the electric fluid.-Although we have already made some passing remarks on this subject, the following detail will, we doubt not, be found interesting to the admirers of the electrical science. Several electricians of distinguished merit have made experiments on the velocity of the electric fluid; those to which we

here particularly refer were made under the direction and superintendence of Mr. Watson, who, as an eye-witness of them, drew up the account to lay before the Royal Society.

391. The first attempt of these electricians was to convey electric shocks across the river Thames, availing themselves of the water of the river as one part of the circuit through which the charge was to pass. This they accomplished on the 14th and 18th of July, 1747, by fastening a wire all along Westminster-bridge, at a considerable height above the water. One end of this wire communicated with the coating of a charged phial, the other being held by an observer, who, in his other hand, held an iron rod, which he dipped into the river. On the opposite side of the river stood a gentleman, who likewise dipped an iron rod in the river with one hand; and in the other held a wire, the extremity of which might be brought into contact with the wire of the phial.

392. When the discharge was made, the shock was distinctly felt by the observers on both sides of the river, but more sensibly by those who were stationed on the same side with the machine; part of the electric fire having gone from the wire down the moist stones of the bridge, thereby making several shorter circuits to the phial, but still all passing through the gentlemen who were stationed on the same side with the machine. This was, in a manner, demonstrated by some persons feeling a sensible shock in their arms and feet, who only happened to touch the wire at the time of one of the discharges, when they were standing upon the wet steps which led to the river. In one of the discharges made upon this occasion, spirits were kindled by the fire which had gone through the river. The gentlemen made use of wires in preference to chains, as communicating a stronger degree of electricity.

393. Their next attempt was to cause the electrical fluid to make a circuit of two miles, at the New River at Stoke Newington. This they performed on the 24th of July, 1747, at two places; at one of which the distance by land was 800 feet, and by water 2000: in the other, the distance by land was 2800 feet, and by water 8000. The disposition of the apparatus was similar to what they had before used at Westminsterbridge, and the effect answered their utmost expectations. But as, in both cases, the observers at both extremities of the chain, which terminated in the water, felt the shock as well when they stood with their rods fixed into the earth twenty feet from the water, as when they were put into the river; it occasioned a doubt, whether the electric circuit was formed through the windings of the river, or a much shorter way, by the ground of the meadow; for the experiment plainly showed that the meadow-ground, with the grass on it, conducted the electricity very well.

394. From subsequent experiments they were fully convinced that the electricity had not in this case been conveyed by the water of the river, which was two miles in length, but by land, where the distance was only one mile; in which space, however, the electric matter must neces

sarily have passed over the New River twice, and have gone through several gravel-pits, and a large stubble-field.

395. On the 28th of July, they repeated the experiment at the same place, with the following variation of circumstances :-The iron wire was, in its whole length, supported by dry sticks, and the observers stood upon original electrics; the effect was, that they felt the shock much more sensibly than when the conducting-wire had lain upon the ground, and when the observers had likewise stood upon the ground, as in the former experiment. Afterwards, every thing else remaining as before, the observers were directed, instead of dipping their rods into the water, to put them into the ground, each 150 feet from the water. They were both smartly struck, though they were distant from each other above 500 feet.

396. Their next object was to determine whether the electric virtue could be conveyed through dry ground; and, at the same time, to carry it through water to a greater distance than they had done before. For this purpose they pitched upon Highbury Barn, beyond Islington, where they carried it into execution on the 5th of August, 1747. They chose a station for their machine almost equally distant from two other stations, for observers upon the New River, which were somewhat more than a mile asunder by land, and two miles by water.

397. They had found the streets of London, when dry, to conduct very strongly for about forty yards; and the dry road at Newington about the same distance. The event of this trial answered their expectations. The electric fire made the circuit of the water, when both the wires and the observers were supported upon original electrics, and the rods dipped into the river. They also both felt the shock, when one of the observers was placed in a dry gravelly pit, about 300 yards nearer the machine than the former station, and 100 yards distant from the river: from which the gentlemen were satisfied, that the dry gravelly ground had conducted the electricity as strongly as water.

398. From the shocks which the observers received, when the electric power was conducted upon dry sticks, they were of opinion, that, from the difference of distance simply considered, the force of the shock, as far as they had yet experienced, was very little if at all impaired. When they stood upon electrics, and touched the water on the ground with the iron rods, the shock was always felt in their arms or wrists; when they stood upon the ground, with their iron rods, they felt the shock in their elbows, wrists, and ankles; and, when they stood upon the ground without rods, the shock was always felt in the elbow and wrist of that hand which held the conducting Fire, and in both ankles.

399. The last investigation which these gentlemen made on this subject, and which required all their sagacity and address in the conduct of it, was to try whether the electric shock was perceptible at twice the distance to which they had before carried it, in ground perfectly dry, and where no water was near; and also to distinguish, if possible, the respective velocity of elec

tricity and sound. For this purpose they fixed upon Shooter's Hill, and made their first experiments on the 14th of August, 1747; a time when, as it happened, but one shower of rain had fallen during five preceding weeks. The wire communicating with the iron rod which made the discharge, was 6732 feet in length, and was supported all the way upon baked sticks: as was also the wire which communicated with the coating of the phial which was 3868 feet long, and the observers were distant from each other two miles.

400. The result of the explosion demonstrated to the satisfaction of the gentlemen present, that the circuit performed by the electric matter was four miles, viz. two miles of wire and two of dry ground, the space between the extremities of the wires; a distance which, without trial, as they justly observed, was too great to be credited. A gun was discharged at the instant of the explosion and the observers had stop-watches in their hands, to note the moment when they felt the shock: but as far as they could distinguish, the time in which the electric matter performed that vast circuit might have been instantaneous. In all the explosions where the circuit was made of considerable length, it was observed that though the phial was very well charged, yet that the snap at the gun barrel, made by the explosion was not near so loud as when the circuit was formed in a room: so that a by-stander, says Dr. Watson, though versed in these operations, would not imagine from seeing the flash, and hearing the report that the stroke at the extremity of the conducting wire could have been considerable; the contrary whereof, when the wires were properly managed, he says, always happened.

401. Still, these philosophers were desirous to ascertain the absolute velocity of electricity at a certain distance; because though in the last experiment, the time of its progress was certainly very small, if any, they were desirous of knowing, small as that time might be, whether it was measurable: and Dr. Watson had contrived an excellent method for that purpose. Accordingly, on the 5th of August, 1748, the gentlemen met once more at Shooter's Hill; when it was agreed to make an electric circuit of two miles, by several turnings of the wire in the same field. The middle of this circuit they contrived to be in the same room with the machine, where an observer took in each hand one of the extremities of the wires, each of which was a mile in length. this excellent disposition of the apparatus, in which the time between the explosion, and the shock might have been observed to the greatest exactness, the phial was discharged several times; but the observer always felt himself shocked at the very instant of making the explosion. Upon this the gentlemen were fully satisfied, that through the whole length of this wire, which was 12,276 feet, the velocity of the electric matter was instantaneous.

In

402. Notwithstanding all this surprising velocity, it is certain, that both sides of a charged phial may be touched so quickly, even by the best conductors, that all the electric matter had not time to make the circuit, and the phial will

remain but half discharged. If the upper plate of an electrophorus also is very suddenly touched with the finger, or any other conductor, a very small spark will be obtained on lifting it up; though a very strong one would be got if the finger was kept longer upon it. But how this seeming slowness can be reconciled with the immeasurable velocity above mentioned, does not appear. It is certain, indeed, that this fluid is considerably resisted in its passages through or over every substance. It will even prefer a short passage in the air where it is violently resisted to one along a wire of very great length; but here, as in every other case, it seems to divide its force, and to break through several different passages at once.

403. The amazing velocity of the electric fluid has recently given rise to some speculations on the subject of constructing electrical telegraphs; this idea, however, appears altogether chimerical, as has been proved by some experiments made by professor Barlow, of the Royal Military Academy. By employing wires of different lengths up to 840 feet, and measuring the energy of the electric action by the deflection produced in the magnetic needle, he found that the intensity rapidly diminishes, and very nearly as the inverse square of the distance. Mr. Barlow also ascertained that the effect was greater with a wire of a certain size than with a finer one; but at the same time, that no advantage was gained by increasing the diameter beyond a certain limit.

404. We have thus gone through what we consider as the essentials of electricity in general; we have omitted several things which we consider as being now obsolete, and some also that are of too trifling a nature to deserve a place in any work pretending to respectability. It has been our aim to produce the most useful information on every part of the subject, and to give the whole as much interest and life as the nature of a subject, purely philosophical, would admit. It may, however, be adviseable, prior to closing the present article, to furnish our readers with the latest facts in the science of electricity, and many that follow are discoveries that are due to the period that we are now writing.

ON PARATONNERRES, OR CONDUCTORS OF
LIGHTNING.

405. A very interesting report on the subject of paratonnerres, has been presented to the Royal Academy of Sciences by M. Gay Lussac. The paper is divided into two parts; one theoretical, and the other practical, and the information contained in it may be regarded as the most perfect we possess on the subject.

406. The theoretical part is introduced with some general observations on electric matter, and of conductors; that its velocity is at the rate of about 1950 feet per second; that. it penetrates bodies, and traverses their substance, with unequal degrees of velocity; that the resistance of a conductor increases with its length, and may exceed that which would be offered by a worse but shorter conductor; and that conductors of small diameter conduct worse than those of larger. The electric matter also tends always to spread itself over conductors, and to assume a

state of equilibrium in them, and becomes divided among them in proportion to their form, and principally to their extent of surface; and that hence a body that is charged with the fluid being in communication with the immense surface of the earth, will retain no sensible portion of it.

407. Gay Lussac defines a paratonnerre to be a conductor which the electric matter prefers to the surrounding bodies, in order to reach the ground, and expand itself through it; and commonly consists of a bar of iron elevated on the buildings it is intended to protect, and descend without any divisions or breaks in its length, into water or moist ground. When a paratonnerre has any breaks in it, or is not in perfect communication with a moist soil, the lightning, having struck it, flies from it to some neighbouring body, or divides itself between the two, in order to pass more rapidly into the earth.

408. The most advantageous form that can be given to the extremity of a paratonnerre is that of a sharp cone, and the higher it is elevated in the air, other circumstances being equal, the more its efficacy will be increased, as is proved by the experiments of M.M. de Romas and Charles.

409. It has not been accurately ascertained how far the sphere of action of a paratonnerre extends; but, in several instances, the more remote parts of large buildings on which they have been erected, have been struck by lightning at the distance of three or four times the length of the conductor from the rod. According, however, to the opinion of Charles, a paratonnerre will effectually protect from lightning a circular space, whose radius is twice that of the height of the conductor. By increasing, therefore, the altitude of a conductor, the space als which it will protect is augmented in pr portion.

410. A current of electric matter, whether luminous or not, is always accompanied by heat, the intensity of which depends on the velocity of the current. This heat is sufficient to make a metallic wire red hot, or to fuse or disperse it, if sufficiently thin; and hence we may perceive the absurdity of some attempts which have been lately made, to protect ships, by thin slips of copper nailed to the masts. The heat of the electric fluid scarcely raises the temperature of a bar of metal, on account of its large mass; and no instance has yet occurred of an iron bar, of rather more than half an inch square, or of a cylinder of the same diameter, having been fused, or even heated red hot by lightning. A rod of this size would, therefore, be sufficient for a paratonnerre; but, as its stem should rise from fifteen to thirty feet above the building, it would not be of sufficient strength at the base to resist the action of the wind, unless it were made much thicker at that part. An iron bar, about three-quarters of an inch. is sufficient for the conductor of the paratonnerre.

411. According to Gay Lussac, a paratonnerre consists of two parts, the stem which projects in the air above the roof, and the conductor,

which descends from the foot of the stem to the ground. The stem he proposes to be a square bar of iron, tapering from its base to the summit, in form of a pyramid, and for a height of from twenty to thirty feet, which is the mean length of the stems placed on large buildings; the base should be about two inches and a half square. Iron being very liable to rust by the action of air and moisture, the point of the stem would soon become blunt; and therefore, to prevent it, a portion of the top, about twenty inches in length, should be composed of a conical stem of brass or copper, gilt at its extremity, or terminated by a small platina needle, two inches long. Instead of the platina needle, one of standard silver may be substituted, composed of nine parts of silver, and one of copper. The platina needle should be soldered with a silver solder to the copper stem; and to prevent its separating from it, which might sometimes happen, notwithstanding the solder, it should be secured by a small collar of copper. The copper stem is united to the iron one, by means of a gudgeon, which screws into each; the gudgeon is first fixed in the copper stem by two steady pins at right angles to each other, and is then screwed into the iron stem, and secured there also by a steady pin.

412. The conductor should be about threequarters of an inch square, and, as before stated, reach from the foot of the stem to the ground. It should be firmly united to the stem, by being tightly jammed between the two ears of a collar, by means of a bolt. The conductor should be supported parallel to the roof, at about six inches distance from it, by forked stanchions, and after turning over the cornice of the building, without touching it, should be brought down the wall, and to which it should be fastened by means of cramps. At the bottom of the wall, it is bent at right angles, and carried in that direction twelve or fifteen feet, when it turns down into a well.

413. Since iron buried in the ground in immediate contact with moist earth soon becomes covered with rust, and is by degrees destroyed, the conductor should be placed in a trough filled with charcoal, in the following manner. Having made a trench in the earth, about two feet deep, a row of bricks is laid on their broad faces, and on them others on edge; a stratum of bakers' ashes (bra se de boulanger) is then strewed over the bottom bricks, about two inches thick, on which the conductor is laid, and the trough then filled up with more ashes, and closed by a row of bricks laid along the top. Iron thus buried in charcoal, will undergo no change in thirty years. After leaving the trough, the conductor passes through the side of the well before alluded to, and descends into the water to the depth of at least two feet below the lowest water line. The extremity of the conductor usually terminates in two or three branches, to give a readier passage to the lightning into the water If there be no well at hand, a hole must be made in the ground, with a six-inch auger, to the depth of about ten or fifteen feet, and the conductor passed to the bottom of it, placing it carefully in the centre of the hole, which is then to be filled up

with bakers' ashes, rammed down as hard as possible, all round the conductor. In a dry soil, or on a rock, the trench to receive the conductor should be at least twice as long as that for a common soil, and even longer, if thereby it be possible to reach moist ground. Should the situation not admit of the trench being much increased in length, others, in a transverse direction, should be made, in which small bars of iron, surrounded by ashes, are placed and connected with the conductor. In general, the trench should be made in the dampest, and consequently lowest spot near the building, and the water gutters made to discharge their waters over it, so as to keep it always moist. Too great precautions cannot be taken to give the lightning a ready passage to the ground, for it is chiefly on this that the efficacy of a paratonnerre depends.

414. As iron bars are difficult to bend according to the projections of a building, it has been proposed to substitute metallic ropes in their stead. Fifteen iron wires are twisted together, to form one strand, and four of these form a rope, about an inch in diameter. To prevent its rusting, each strand is well tarred, separately, and, after they are twisted together, the whole rope is tarred over again with great care. Copper or brass wire is, however, a better material for their construction than iron. If a building contain any large masses of metal, as sheets of copper or lead on the roof, metal pipes and gutters, iron braces, &c., they must all be connected with the paratonnerre, by iron bars of about half an inch square, or something less. Without this precaution, the lightning might strike from the conductor to the metal (especially if there should be any accidental break in the former), and occasion very serious injury to the building, and danger to its inhabitants.

415. Paratonnerres for Churches. — For a tower, the stem of the paratonnerre should rise from fifteen to twenty-four feet, according to its area; the domes and steeples of churches, being usually much higher than the surrounding objects, do not require so high a conductor as buildings with extensive flat roofs. For the former, therefore, their stems, rising from three to six feet above the cross or weather-cock, will be sufficient, and being light they may easily be fixed to them without injuring their appearance, or interfering with the motion of the vane.

416. Paratonnerres for Powder-Magazines.-These require to be constructed with the greatest care. They should not be placed on the buildings, but on poles at from six to ten feet distance. The stems should be about seven feet long, and the poles of such a height, that the stem may rise from fifteen to twenty feet above the top of the building. It is also advisable to have several paratonnerres round each magazine. If the magazine be in a tower, or other very lofty building, it may be sufficient to defend it by a double copper conductor, without any paratonnerre stem. As the influence of this conductor will not extend beyond the building, it cannot attract the lightning from a distance, and will yet protect the magazine, should it be struck.

417. Paratonnerres for Ships.-The stem of a paratonnerre for a ship, consists merely of a