a term used as well to signify the dignity of, as the territories belonging to, any of the electors of Germany; such are Bavaria, Saxony, &c. THE word ELECTRICITY signifies, in general, the effects of a very subtile fluid matter, different in its properties from every other fluid we are acquainted with. This fluid is capable of uniting with almost every body, but unites more readily with some particular bodies than with others; its motion is amazingly quick, is regulated by peculiar laws, and produces a vast variety of singular phenomena, the principal of which shall be enumerated in this article.

As we are entirely ignorant of the nature of the electrical fluid, it is impossible to define it but by its principal properties: that of repelling and attracting light bodies, is one of the most remarkable. The ancients were only acquainted with this property in amber. William Gilbert, a native of Colchester, and physician at London, in his treatise De Magnete, in the year 1600, was the first person who discovered, that sulphur, wax, resinous substances, glass, and precious stones, when dried and rubbed a little, were endowed with the same property of attracting and repelling straws and other light substances. Sir Francis Bacon, in his physiological remains, gives a catalogue of electrical bodies; but it differs in nothing worth mentioning from that of Gilbert. Mr Boyle, about the year 1670, made some addition to the catalogue of electric substances; but all his experiments on this subject relate only to a few circumstances attending the simple property of electric attraction: he had never seen the electric light, and little imagined what astonishing effects would be afterwards produced by this wonderful power.

Contemporary with Mr Boyle was Otto Guericke, burgomaster of Magdeburg, and inventor of the air-pump, who was likewise one of the first improvers of electricity. He made his experiments with a globe of sulphur, which he mounted on an axis, and whirled it in a wooden frame, rubbing it at the same time with his hand. He first discovered, that a body once attracted by an excited electric was repelled by it, and not attracted again till it had been touched by some other body: that bodies immersed in electric atmospheres are themselves electrified: that threads suspended within a small distance of his excited globe, were often repelled by his finger brought near them: that a feather, repelled by the globe, always turned the same face towards it, like the moon with respect to the earth; and that the excitation of his globe produced both light and sound, though in a very considerable degree. A much finer electric light was afterwards observed by Dr Wall, and an account of it was published in the Philosophical Transactions: Dr Wall likewise compares the light and the cracking of his excited amber to thunder and lightening.

Sir Isaac Newton, in 1675, was the first who discovered that excited glass attracted light bodies on the side opposite to that on which it was rubbed.

After Gilbert, Boyle, and Otto Guericke, Mr Hawkesbee, in his Physico-mechanical Experiments, published in the year 1709, distinguished himself by his experiments and discoveries in electricity. He first discovered the electric power of glass, the light proceeding from it, and the noise occasioned by it, together with a variety of phenomena relating to electric attraction and repulsion: Indeed little was added to his observations, till the discovery of a plus and minus electricity by Dr Watson and Dr Franklin about the year 1746, and the farther illustration of that doctrine by Mr Canton.

From the year 1730 to the 1746, the writers on electricity are so numerous, and their experiments so many and various, that a volume would be insufficient for their history. We shall therefore endeavour, in the first place, To give a short and connected view of the nature and principles of electricity, so far as they have hitherto been unfolded, without mentioning the persons to whom we are indebted for any particular discovery: And, in the second place, Give a description of electrical machines; with a selection of a few of the most curious and useful experiments, which the reader may easily understand after having made himself acquainted with the general principles.

It has been asserted, that all bodies, provided they be heated to a certain degree, and rubbed for a long time, will discover themselves to be possessed of the property of attracting and repelling light substances. However, metals of all kinds, although ever so much heated, or rubbed, or polished, never discover the least signs of electrical attraction; and consequently are excepted from the general rule, as well as water and other fluids, which cannot be subjected to the necessary treatment. Although most bodies, by being heated and rubbed, discover more or less of electrical attraction; yet, as some of them possess this property in a more eminent degree, and with less labour, this circumstance has suggested a division of bodies into two classes, according as they are more or less susceptible of electricity.

The first class comprehends those bodies which receive and collect the electrical matter most easily, and in greatest quantity, after being a little rubbed and heated: these bodies are called electrics, or non conductors: such as,

1. Diamonds of all kinds; the ruby, the sapphire; the emerald, the opal, the amethyst, the topaz, the beryl, the garnet, rock crystal, &c.

2. Glasses, and all vitrified bodies, enamels of all colours, porcelain, glasses of antimony, of lead, &c.

3. Balms, resins of all kinds, wax, &c.

4. Bituminous bodies, sulphur, amber, asphaltum, &c.

5. Certain animal productions; as silk, feathers, wool, hairs, and bristles, &c.

The second class comprehends those bodies which either do not at all collect the electrical matter by friction, or or in a very inconsiderable degree: such bodies are called non-electrics, or conductors, viz:

1. Water, and all aqueous and spirituous liquors, which are incapable of being thickened, and subjected to friction.

2. All metals, perfect and imperfect, and the greatest part of minerals; as, the load-stone, antimony, zinc, bismuth, the agat, the jaiper, marble, free-stone, slate, &c.

3. All living creatures, excepting their hair. To which may be added most animal-substances; as leather, parchment, bone, ivory, horn, shells, &c.

4. Trees and plants of all kinds; thread, ropes, linen-cloth, paper, &c.

These two classes of bodies have been called by the name of electrics and non-electrics; but as the electrical matter is not contained in the electrical bodies themselves, but collected by them from the earth; and as non-electric allow the electrical matter to penetrate and flow through them, or to spread equally on their surfaces, the terms conductor and non-conductor are more proper. Metals and water are the only perfect conductors; other bodies conducting only as they contain a mixture of these, without more or less of which they will not conduct at all.

Although conductors cannot be electrified by heat or friction, they may be charged with electricity, or made non-conductors, by an easy operation; but then they retain this property no longer than they are kept from communicating with other conductors. A bar of iron will become an electric or non-conductor, by being suspended by a silk cord, or laid upon a piece of rosin or other non-conductor, and at the same time having one end of it in contact with a well rubbed glass tube, or globe. In the same manner, water and metals of all kinds may be charged with electricity.

It is absolutely necessary, in exciting electricity by friction, that the glass, or other body, be perfectly dry; the least moisture destroys, or at least diminishes the effect. A moist atmosphere, a burning candle, &c. are extremely unfavourable in making electrical experiments.

Hollow glass globes, of about a foot diameter, and the 16th part of an inch thick, are now used in place of tubes, because it lessens the labour of friction, and accumulates a greater quantity of electrical matter. This globe is turned rapidly by a large wheel like those used by the cutlers. When the globe is rubbed, it soon acquires a considerable degree of electrical virtue, which is discovered by light bodies, at the distance of two or three feet, flying towards it. In approaching the globe with the hand or face, you will likewise feel the electric matter surrounding it like a gentle breeze of wind. These subtle emanations continue to be diffused round the globe as long as the friction is continued; and, when the friction is stopped, they gradually diminish, till they are no longer perceptible. The application of non-conductors to the globe does not diminish the electrical matter: on the contrary, the application of conductors almost instantly annihilates the whole quantity previously collected by the friction. But this effect is not produced, unless when the conductor at the same time has a communication with the floor or earth where the machine stands. For, as above observed, if the conductor has no communication with the earth, it charges with electricity, and becomes a non-conductor. But non-conductors, as a piece of glass, sulphur, or wax, though they do not diminish the virtue of the globe, yet they do not acquire, like iron, &c. the property of attraction or repulsion. Hence it appears, that the electrical matter passes freely along conductors, and dissipates in the earth; but, on the contrary, that non-conductors do not receive any matter from the globe, and are incapable of transmitting it. The following experiments will make this more plain.

1. If a piece of iron be placed on a glass standard, and unconnected with any other body, as soon as the electrical matter is communicated to it, it attracts and repels pieces of gold leaf or other light bodies, and preserves this virtue even for some minutes after its communication with the globe is cut off. But, if a piece of glass, rosin, or any other non-conductor, be placed in the same circumstances, they do not discover any such effect.

2. If a person touches the piece of iron above mentioned with his hand, no friction is capable of making it attract or repel, or exhibit any marks of electricity. The same thing happens, if a chain of any metal touch the iron, and at the same time has a communication with the ground. In both these cases, the electrical matter passes along the iron, and dissipates in the earth.

3. In place of touching the piece of iron with the finger, if a piece of amber, wax, or any other non-conductor be applied to it, the communication with the earth being interrupted by the non-conductor, the iron, in that case, retains the electrical matter as before.

From these experiments we learn, that metals and other conductors receive the electrical matter, and transmit it to other conductors till it diffuses and is lost in the earth; but that, if wax, glass, or any other non-conductor, be applied to the conductor, the motions of the matter is instantly stopt, and accumulates and charges the conductor, at the same time that the non-conductor itself is not all affected. It is for this reason that conductors, as silk-cords, hair-ropes, &c. are always employed to suspend or support such bodies as we want to charge with electricity.

The following experiments will throw further light on this subject.

1. If a man stands upon a piece of rosin about five inches in diameter, and seven or eight inches thick, touching softly the globe, while the operator is rubbing it, his whole body, in a few seconds, will be charged with electrical matter; and the following phenomena will take place.

1. His loose hand, and indeed every part of his body, will mutually attract and repel light bodies at the distance of three or four feet;

2. All conductors which he takes in his hand, will become electrified in the same manner with himself, provided they touch nothing else, or be supported upon non-conductors: and this communication to other conductors, let Let their number and extension be ever so great, instead of diminishing the electrical virtue in the body of the man, will rather augment its strength and quantity.

3. If this person gives his hand to another likewise standing on a similar piece of rosin, he too will be charged with electrical matter; and the same thing will happen to any number of persons, provided they stand upon rosin, and communicate with one another by an iron chain or other conductor. But the whole company will instantaneously lose the whole of their electrical virtue, if any non-electrified person touch a single man, or if there be any communication between one of them and a conducting substance.

4. If the first man removes his hand from the globe, and at the same time keep his former station, he and all the rest will preserve the power of attracting and repelling light substances for some time; but it gradually diminishes, till its effects totally disappear.

5. If a non-electrified person puts his hand near the first man's face, he will feel a kind of atmosphere surrounding the electrified person; if he advances his hand still nearer any part of the face, for example the nose, both the point of the finger and the nose will appear luminous in the dark: Lastly, if he touches the nose, a spark of fire instantly explodes with a crack, and strikes both parties equally with a shock more or less painful in proportion as the electrified person was charged. It is by the explosion of this spark, that the electrical matter instantly transmits itself from one body to another.

6. When we approach near an electrified person, we perceive an extraordinary smell proceeding from his body, similar to that of the phosphorus of urine.

II. An iron wire, 12,000 feet in length, was suspended about five feet from the ground by silk cords; one end of it was connected to the globe of an electrical machine, and at the other a lead ball was hung in order to perceive when the matter reached it.

1. After five or six turns of the wheel, the matter had passed along the whole wire, and communicated its virtue to the ball, which instantly attracted and repelled light bodies.

2. As this ball was equally electrified with every part of the wire, it is probable that the electric matter would instantly pervade a wire of a still greater length, provided we had a proper apparatus for the purpose.

3. Several metals and other conductors were substituted in place of the ball, and all received the electricity in the same manner.

4. The ball and other non-conductors, when touched with the finger, gave a luminous spark and as smart a shock, as when the end of the wire next the globe was touched.

5. All these effects instantly ceased whenever any person not electrified touched any part of the wire, and commenced again a few seconds after his hand was withdrawn.

6. The same effects are produced, though with more difficulty, when hair or woolen ropes were substituted in place of the silk ones: But they were entirely stopped by hemp-ropes, or when the silk ones were wetted.

7. When a hempen rope was substituted in place of the wire, the ball at the end of it was electrified with greater difficulty than when it hung at the wire, especially when the rope was dry; but when the rope was wet, the matter passed with more ease.

8. When, in place of the wire, a dry silk cord, or long glass tube, were used, they received but a very small quantity of the matter, which was not perceptible in the glass tube above 12 feet, nor in the cord above 25, beyond the globe.

9. When the wire was cut in several places, and the cut ends kept at the distance of somewhat less than a foot from each, the electrical matter darted through all these interruptions, and appeared in the ball at the furthest end. A strong blast made by a bellows across one of the interruptions, did not obstruct the passage of the matter; neither did the interposition of a piece of glass, wax, and other non-conductors: but all conductors, as the hand of a man, the point of a sword, and even a moist vapour, obstructed its course towards the ball.

10. When a man stood on a piece of rosin, and put a point of a sword in one of the interruptions, he was instantly filled with the matter, although neither he nor the sword touched the wire: neither was the course of the matter towards the ball obstructed by the interposition of the sword.

11. When a ring of brass wire, about three feet in diameter, was suspended in a vertical direction, and the iron wire was made to pass nearly through its centre, without touching any part of the circumference, the ring, in whatever part of the wire it was tried, was sensibly electrified. This shows that the electrical matter expands to a considerable distance on all sides of the electrified body.

12. The same iron wire suspended by silk cords was extended 6000 feet, (just one half of its length), in a straight line; the other half was turned back in a parallel direction towards the globe, leaving about nine or ten inches of interval between the two halves of the wire: Each extremity of the wire was supported by a dry silk cord about seven or eight feet from the globe, and the lead ball was hung at one of them. An iron chain was then fixed with another silk cord above the globe, in order to receive the matter at one of its extremities; the other end of the chain was fixed to a rod of glass about five feet long, in such a manner that the matter received from the globe might be transmitted at pleasure to the wire, by applying the end of the fixed chain to the glass rod. Matters being thus prepared, after five or six turns of the wheel, the chain was applied to one end of the wire; at the same instant the ball at the other end attracted and repelled bits of gold-leaf. The same experiment was repeated, and a finger applied to the ball, and a spark issued out, and a shock was received at the very instant that the chain was applied to the other end of the wire: A spark likewise proceeded from the chain, which afforded an easy opportunity of discovering that the two sparks were perfectly synchronous.

From these experiments it appears,

1. That the electrical matter communicates itself to all non-electrics, or conductors, whatever be their bulk or extension.

2. That 2. That the quantity of this matter diffused is always in proportion to the magnitude and extension of the bodies into which it passes; and that it is uniformly diffused, no part of the body retaining more than any other part.

3. That, after being thus communicated to any body, it escapes with equal facility, as soon as it finds a communication with the earth.

4. That small interruptions in the continuity of electrified bodies do not interrupt the motion of the electrical matter.

5. That the motion of the electrical matter is so amazingly swift, that it runs over a space of 12,000 feet in an undefinable instant of time.

6. That it moves with equal rapidity either backward or forward, upon the application of a conductor.

7. Lastly, That an indefinitely large quantity of this matter may be accumulated by applying the globe or tube to conducting bodies of very large dimensions. Of late other methods of condensing a large quantity of electrical matter into a small space have been invented, as will afterwards appear when we come to treat of the Leyden phial.

The attraction and repulsion of light bodies, is the first thing that discovers to us the presence of the electrical matter. This motion is always reciprocal: If the electrified body be lighter than the conductor, and both are at liberty, the motion of the former is quicker than that of the latter; if the one be fixed, and the other at liberty, the unfixed one constantly goes to the one that is fixed, and, at the same, takes the shortest road. The following experiments will illustrate these motions:

1. Present an electrified tube to small pieces of gold-leaf placed on a well-polished plate of copper, they will instantly fly towards the tube.

2. Suspend an electrified tube by two silk cords; take a piece of gold-leaf, and, holding it firm betwixt your fingers, bring it near the tube; and the tube will be attracted and move toward the leaf.

3. If an electrified person, standing on a piece of rosin, holds in his hand a plate of copper, upon which pieces of gold-leaf are placed; and another person, who is not electrified, holds his finger above the plate; the gold-leaf will instantly rise from the plate, and fly towards his finger.

4. Lastly, If two balls of gilt paper be suspended six inches asunder, the one by a silk thread three feet in length, and the other by a small silver wire of the same length; when the ball suspended by the silk thread is electrified by the tube, both balls advance with equal quickness towards one another, though only one of them was electrified.

The most favourable circumstances for exhibiting the attraction of light bodies are the following:

1. They should be perfect conductors. 2. They ought to be of a small size. 3. They should be supported by a non-conductor, and raised four or five feet from the ground. 4. No other non-conductor should be nearer the bodies than the tube with which the experiments are making; otherwise the attraction will be disturbed.

Repulsion generally succeeds attraction; that is, a piece of gold-leaf is no sooner attracted by the tube than it is repelled and driven off from it. This repulsion is not very perceptible when the tube is slightly electrified; but, when the electricity is brisk, the gold-leaf never fails to be repelled as soon as it has touched the tube. Again, if the electricity be very strong, the gold-leaf, though strongly attracted by the tube, never touches it; the repulsive power beginning to operate two or three inches before the leaf reaches the tube: from that instant the leaf is electrified; and, when it begins to be repelled, it has acquired as dense an electrical atmosphere as the tube; it then flies off, and remains suspended above the tube until it loses the electric virtue it had acquired, either by the moist vapours in the air, or till it loses it suddenly by touching some conductor. Hence it appears, that attraction precedes repulsion, only because it is necessary that the pieces of gold-leaf should acquire as dense an atmosphere as that of the globe before they can be repelled by it.

When the tube has repelled a piece of gold leaf, if another tube, nearly equally electrified, be suddenly substituted in place of the former tube, the leaf will continue to be repelled at an equal distance. But if the substituted tube be much less electrified than the original one, the leaf will be attracted by that tube.

When two or more pieces of gold leaf are presented at the same time to a well electrified tube, they are all equally attracted and repelled; but then they mutually repel one another, so that it is impossible to make any two of them join; and the distance at which they repel one another is equal to the distance to which each of them were repelled from the tube.

If a circular piece of gold-leaf, cut into small fringes to near the centre of the leaf, be presented to an electrified tube, it will first be attracted, and then repelled: in the time of repulsion, all the fringes repel each other, and diverge more or less in proportion to the strength of electricity in the tube.

If a small metal vessel filled with water, and furnished with a capillary siphon, having the longer leg hanging over the outside of the vessel, be touched with an electrified iron rod; the water, which could not run out of the siphon but drop by drop, will instantly fly out a tone jet, and divide itself into very fine threads; and these threads continue sometimes suspended in the air, repelled from each other to a considerable distance.

From these instances of attraction and repulsion it appears,

1. That light bodies are attracted by electrified substances until they are equally electrified by communication, and until they acquire as dense an atmosphere as the electrified substances themselves. 2. That, from the moment they acquire this atmosphere, attraction ceases and repulsion begins. 3. That no repulsion takes place but between bodies electrified. 4. That repulsion continues only as long as the density of the two atmospheres are equal; that it ceases whenever the one or the other is diminished; that a new attrac- traction commences, and continues till an equality in the atmosphere is again restored; and that, immediately upon this, a new repulsion takes place.

5. That repulsion may subsist betwixt two bodies which have never mutually been attracted, provided their atmospheres be equally dense.

6. That the distance to which bodies are repelled is always in proportion to the strength of the electricity they contain. This fact first suggested the notion of an electrometer, or a machine for measuring the different degrees of electricity.

Of Electrical Machines and Apparatus.

The improvement of electrical machines has kept pace with the improvements in the science. While nothing more than electrical attraction and repulsion was known, every phenomenon might be exhibited by means of a piece of amber, sealing wax, or glass, which the philosopher rubbed against his coat, and presented to bits of paper, feathers, and other light bodies.

To give a greater degree of friction to electric substances, Otto Guericke and Mr Hawkesbee contrived to whirl sulphur and glass in a spherical form. The first conductors were nothing more than hempen-cords supported by silk lines. In place of these, bars of metal, or gun-barrels, were soon substituted; and a rubber was employed to supply the place of a human hand. The discovery of the Leyden bottle, (to be afterwards described) occasioned more additions to the electrical apparatus; and the discoveries of Dr Franklin have made proportional additions necessary. No philosopher can now be satisfied, if he be not able to supply a conductor from the clouds, as well as from the friction of his glass globes or tubes.

Although globes or cylinders are now of the most extensive use in electrical experiments, glass-tubes are still most convenient for several purposes; they should be about three feet long, and as wide as a person can conveniently grasp, (Plate LXXXIII. fig. 1 a.) The thickness of the glass is not material; perhaps the thinner they are the better, if they can bear sufficient friction.

The best rubber for a smooth glass is the rough side of black oiled silk, especially when a little amalgam of mercury or other metal is put upon it.

Glass-globes are in general preferable to cylinders. The globe should have its neck inclosed in a pretty deep brafs cap, ending in a dilated brim, of about half an inch broad, if the globe be a large one. It has not been determined what kind of glass is the best; but flint is commonly used. Perhaps globes of twelve or thirteen inches diameter are the best size.

The best rubbers for globes are made of red bafil skins, particularly the neck-part of them, where the grain is more open, and the surface somewhat rougher. That the rubber may press the globe equally, it should be put upon a plate of metal bent to the shape of the globe, and stuffed with any thing that is pretty soft: brass is good; and if the stuffing be a conductor, as flax, it will be better than if it be a non-conductor, as hair or wool. It should rest upon a spring, to favour any inequality there may be in the form of the globe. There should be no sharp edges or angles about the rubber; for that would make the insulation of it ineffectual. By the insulation of the rubber every electrical experiment may be performed with the twofold variety of positive and negative, and a conductor be made to give and take fire at pleasure. This insulation is best made by means of baked wood, in the form of a plate, five or six inches in diameter, (g, fig. 2.) interposed between the metallic part of the rubber and the steel spring that supports it. When positive electricity is intended to be produced, a chain (n, fig. 2.) must connect the rubber with the floor; but, when negative electricity is wanted, the chain must be removed, and hung upon the common conductor, while another prime conductor must be connected with the rubber; which will therefore be electrified negatively.

The belt method of collecting the electric fire from the globe seems to be by three or four pointed wires, (m, fig. 2.) two or three inches long, hanging lightly upon the globe, and suspended on an open metallic ring.

The prime conductor should be fixed very steady; whatever be the size of the prime conductor, the extremity of it, or that part which is most remote from the globe, should be much larger and rounder than the rest, (k, fig. 2.); for the effort of the electric matter to fly off is always greatest at the greatest distance from the globe.

The electrician should be provided with Metallic Rods (i, fig. 1.) to take sparks from his conductor for various uses. These should have knobs, larger or smaller in proportion to the curvature of the conductor. If the knob be too small, it will not discharge the conductor at once, but by degrees, and with a less sensible effect; whereas the spark between broad surfaces is thick and strong.

The most formidable part of an electrical apparatus consists in the coated glass that is used for the Leyden experiment. The form of the plate is immaterial with respect to the shock; and, for different experiments, both plates of glass, and jars of various forms and sizes, must be used. For common uses, the most commodious form is that of a jar, as wide as a person can conveniently hold in his hand by grasping, and as tall as it will stand without any danger of falling; perhaps about 3½ inches in diameter, and 8 inches in height. The mouth should be pretty open, that it may be the more conveniently coated on the inside, as well as the outside, with tinfoil. A considerable variety of these jars may be seen in the above Plate, fig. 1. c, d, e, f, g, h, i, j, k.

The method of coating is much preferable to that of putting water or brafs-shavings into the jars, which both makes them heavy, and likewise incapable of being inverted, which is requisite in many experiments. Brass-dust, however, or leaden-shot, is very convenient for small phials. The tinfoil may be put on either with paste, gum water, or bees-wax. To coat the insides of vessels which have narrow mouths, moisten the inside with gum-water, and then pour some brass-dust upon it: Enough will stick to make an exceeding good coating.

In the construction of an ELECTRICAL BATTERY, a number number of small jars are preferable to large ones. If one of them should break by an explosion or any other accident, the loss is less considerable; besides, by means of narrow jars, a greater force (that is, a greater quantity of coated surface) may be contained in less room.

The largest jars are about 17 inches in height, and should not be more than 3 inches in diameter, and of the same width throughout. Thus they may be easily coated both within and without, and a box of moderate size will contain a prodigious force; for the jars being coated within two inches of the top, each will contain a square foot of coated glass. The battery (Plate LXXIII. fig. 3.) consists of 64 jars, each 8 inches long, and 2½ inches in diameter, coated within an inch and a half of the top. The coated part of each is half a square foot; so that the whole battery contains 32 square feet. The wire of each jar has a piece of very small wire twisted about the lower end of it, to touch the inside coating in several places; and it is put through a pretty large piece of cork within the jar, to prevent any part of it from touching the side, which would tend to promote a spontaneous discharge. Each wire is turned round, so as to make a hole or ring at the upper end; and through these rings a pretty thick brass rod with knobs is put, one rod serving for one row of the jars. The communication between these rods is made by laying a chain over them all; this chain is not represented in the plate, lest the figure should appear confused. When only a part of the battery is to be used, the chain should be laid over as many rods as you want rows of jars. The bottom of the box in which all the jars stand is covered with tinfoil and brass dust; and a bent wire touching this tinfoil is put through the box, and appears on the outside, as in the plate. To this wire is fastened whatever is intended to communicate with the outside of the battery, as the piece of small wire in the figure; and the discharge is made by bringing the brass knob to any of the knobs of the battery.

To discover the kind and degree of electricity, many forms of Electrometers have been thought of. Mr Canton's balls A, represented on a glass standing on the stool c, (Plate LXXIII. fig. 1.) serve to discover small degrees of electricity, to observe the changes of it from positive to negative, and to estimate the force of a shock before the discharge. These balls are two pieces of cork, or pith of elder, nicely turned in a lathe to about the size of a small pea, and suspended on small linen threads. These balls repel one another to distances exactly proportioned to the quantity of electricity contained in the vessel or other substance with which they are connected; and by this work the operator knows pretty exactly the force of the charge, and the shock that will be given.

In order to repeat the experiment tending to show that the electric fluid is the same with the matter of lightning, and to make observations on the electricity of the atmosphere, the electrician should be provided with A MACHINE FOR DRAWING ELECTRICITY FROM THE CLOUDS. The best construction of which is the following: On the top of any building erect a pole a, (Plate LXXIV. fig. 2.) as tall as a man can well manage, having on the top of it a solid piece of glass, or baked wood, a foot in length. Let this be covered with a tin or copper vessel (b) shaped like a funnel, to prevent its ever being wetted; above this, let there rise a long slender rod c, terminating in a pointed wire, and having a small wire twisted round its whole length, the better to conduct the electricity to the funnel. From the funnel make a wire (d) descend along the building, about a foot distance from it, and conducted through an open shelf into any room that shall be most convenient for making the experiment. In this room, let a proper conductor be insulated, and connected with the wire coming in at the window. This wire and conductor, being completely insulated, will be electrified whenever there is a considerable quantity of electricity in the air. And notice will be given when it is properly charged, either by Mr Canton's balls hung to it, or by a set of bells disposed in the following manner. Take three bells; suspend the two outermost from the conductor by chains, and that in the middle by a silken string, while a chain connects it with the floor; and hang two small knobs of brass by silken strings, one between each two bells, to serve instead of clappers. In consequence of this disposition, when the two outermost bells, communicating with the conductor, are electrified, they will attract the clappers, and be struck by them. The clappers being thus loaded with electricity, will be repelled, and fly to discharge themselves upon the middle bell. After this, the clappers will be again attracted by the outermost bells; and thus, by striking the bells alternately, a continual ringing may be kept up as long as the operator pleases. In the dark a continual flashing of light will be seen between the clappers and the bells. But when the electrification is very strong, these flashes of light will be so large, that they will be transmitted by the clapper from one bell to the other, without its ever coming to actual contact with either of them, and the ringing will consequently cease.

With regard to the construction of machines for electrical experiments in general, that of Dr Priestly, represented on Plate LXXIII. fig. 2. is perhaps the best. The Frame consists of two strong boards of mahogany, (aa), of the same length, parallel to one another, about four inches asunder, and the lower one is an inch on each side broader than the upper: in the upper board is a groove reaching almost its whole length. One of the pillars b, which are of baked wood, is immovable, being let through the upper board, and firmly fixed in the lower; while the other pillar slides in the groove above-mentioned, in order to receive globes or cylinders of different sizes; but it is only wanted when an axis is used! Both the pillars are perforated with holes at equal distances from the top to the bottom; by means of which, globes may be mounted higher or lower according to their size; and they are made tall, to admit the use of two or more globes at a time, one above another. Four of a moderate size may be used, if two be fixed on one axis; and the wheel has several grooves for that purpose.

If a globe with only one neck be used, as in the Plate, a brass arm, with an open socket c, is necessary to support the axis beyond the pulley; and this part is also contrived to be put higher or lower, together with the brass socket in which the axis stands. The axis d is is made to come quite through the pillar, that it may be turned by another handle without the wheel, if the operator chuses. The frame, being screwed to the table, may be placed nearer to, or farther from, the wheel, as the length of the string requires in different states of the weather. The Wheel is fixed in a frame by itself, by which it may have any situation with respect to the pulley, and be turned to one side, so as to prevent the string from cutting itself.

The Rubber (f) consists of a hollow piece of copper, filled with horse-hair, and covered with a basil-skin. It is supported by a socket, which receives the cylindrical axis, of a round and flat piece of baked wood g, the opposite part of which is inserted into the socket of a bent steel-spring h. These parts are easily separated; so that the rubber, or piece of wood that serves to insulate it, may be changed at pleasure. The spring may be either slipped along the groove, or moved in the contrary direction, so as to give it every desirable position with respect to the globe. It is besides furnished with a screw i, which makes it press harder or lighter on the globe, as the operator chuses.

The Prime Conductor (k) is a hollow vessel of polished copper in the form of a pear, supported by a pillar and a firm basis of baked wood; and it receives the electrical matter by means of a long arched wire or rod of very soft brass l, easily bent into any shape, and raised higher or lower as the globe requires. It is terminated by an open ring, in which are hung some sharp-pointed wires m, playing lightly on the globe when it is in motion. The body of the conductor is furnished with holes and sockets for the insertion of metallic rods to convey the fire wherever it is wanted.

When positive electricity is required, a wire or chain, as represented in the plate (n), connects the rubber with the table or the floor. When negative electricity is wanted, that wire is connected with another conductor, such as that represented, in fig. 1. t; where the conductor in fig. 2. is connected with the table by another wire or chain. If the rubber be made tolerably free from points, the negative power will be as strong as the positive.

The machine, represented Plate LXXV. fig. 1. was a contrivance of Dr Watson's, to whirl four large globes at a time, and unite the power of them all. The construction is so simple, that we need not give any particular description of it, especially after having so fully described that of Dr Priestly.

Of positive and negative Electricity, and the Leyden Phial.

Dr Watson and Dr Franklin first suggested the notion of positive and negative, or plus and minus electricity: several experiments led them to conclude, that every body in nature, and particularly all conducting bodies, possessed a certain quantity of electric matter, and that this natural quantity might be augmented or diminished by being placed in particular circumstances. When a body receives a larger quantity than the natural one, it is said to be electrified plus, or positively; when the natural quantity is diminished, it is said to be electrified minus, or negatively. The following experiments will shew the different circumstances requisite to produce these two kinds of electricity.

1. A person (standing on wax, and rubbing the tube, and another person on wax drawing the fire, they will both appear to be electrified by a person standing on the floor; that is, he will perceive a spark on approaching each of them with his knuckle.

2. But, if the persons on wax touch one another during the exciting of the tube, neither of them will appear to be electrified.

3. If they touch one another after exciting the tube, and drawing the fire as before, there will be a stronger spark between them, than happens between either of them and the person on the floor.

4. After such strong spark, neither of them discover any electricity.

These appearances are explained in the following manner: the electrical fire is supposed to be a common element, of which each of the three persons above-mentioned has his equal share, before any operation is begun with the tube. A, who stands on wax and rubs the tube, collects the electrical fire from himself into the glass; and his communication with all conductors being cut off by the wax, his body is not again immediately supplied. B, who stands likewise on wax, passing his knuckle along near the tube, receives the fire which was collected by the glass from A; and his communication with conductors, or the common stock of electrical matter, being likewise cut off, he retains the additional quantity received. To C, standing on the floor, both appear to be electrified: for he having only the middle quantity of electrical fire, receives a spark upon approaching B who has an over quantity, but gives one to A who has an under quantity. If A and B approach to touch each other, the spark is stronger, because the distance betwixt them is greater: after such touch, there is no spark between either of them and C, because the electrical fire in all is reduced to the original equality. If they touch while electrifying, the equality is never destroyed, the fire only circulating. Hence we say, B is electrified positively, A negatively; or rather B is electrified plus, A minus; and in experimenting, it is common to electrify bodies plus or minus at pleasure. To electrify plus or minus, it is sufficient to know, that the parts of the tube or sphere that are rubbed, do, in the instant of the friction, attract the electrical fire, and therefore take it from the thing rubbing: the same parts immediately, as the friction upon them ceases, are disposed to give the fire they have received to any body that has less. Thus you may circulate it or accumulate it upon, or substract it from any body, as you connect that body with the rubber, or the receiver, the communication in the common stock being cut off.

The great shock from what is called the Leyden phial, was first discovered by Mr Cunaeus, a native of Leyden; but was never so thoroughly understood till Dr Franklin published his experiments with regard to it. A glass phial or jar, filled, till within an inch of the top, with water, brats dust, or other non-conducting substances, was first used; but coating the vessel with tin foil, or brass-dust, as mentioned above in the section concerning the electrical apparatus, was found to answer better.

We shall here give Dr Franklin's account of this phial nearly in his own words, together with the experiments confirming it.

1. While the wire and inside of the bottle are electrified positively or plus, the outside of the bottle is electrified negatively or minus, in exact proportion; i.e., whatever quantity of electrical fire is thrown into the inside, an equal quantity goes out of the outside. To understand this, suppose the natural quantity of electricity in the whole bottle, before the operation begins, is equal to 20; and, at every stroke of the tube, or turn of the globe, suppose a quantity equal to 1 is thrown in; then, after the first stroke, the quantity contained in the wire and inside of the bottle will be 21, and in the outside 19; after the second stroke, the inside will have 22, and the outside 18; and so on, till, after 20 strokes, the inside will have a quantity of electrical fire equal to 40, and the outside none at all; and then the operation ends: for no more can be thrown into the inside, when no more can be driven out of the outside. If more is attempted to be thrown in, it is forced back through the wire, or flies out in loud cracks through the sides of the bottle.

2. The equilibrium of electric matter in the bottle being thus lost, it cannot be restored by any inward communication or contact of the parts: but this must be done by a communication formed without the bottle between the inside and the outside, by some conductor touching or approaching both sides at the same time; in which case the equilibrium is restored with an inexpressible violence and quickness; or, it may be done by touching each side alternately; in which case, the equilibrium is restored by degrees.

3. As no more electrical fire can be thrown into the inside of the bottle, when all is driven from the outside; so, in a bottle not yet electrified, none can be thrown into the inside, when none can get out at the outside; which happens, either when the glass is too thick, or when the bottle is placed in a non-conductor. Again, when the bottle is electrified, but little of the electrical fire can be drawn out from the inside by touching the wire, unless an equal quantity can, at the same time, get in at the outside. Thus, place an electrified bottle on clean glass, or dry wax, and you will not, by touching the wire, get out the fire from the inside: place it on a conductor, and touch the wire, then you will get it out in a short time; but soonest when you form a direct communication as above.

4. The shock to the nerves, or rather convulsion, is occasioned by the sudden passage of the fire through the body, in its way from the inside to the outside of the bottle. The fire takes the shortest course; but it does not appear, that, in order to receive a shock, a communication with the floor is necessary; for he that holds the bottle with one hand, and touches the wire with the other, will be shocked as much, though his shoes be dry, or even standing on wax. And on the touch of the wire (or of the prime conductor, which is the same thing,) the fire does not proceed from the touching finger to the wire, but from the wire to the finger, and passes through the body to the other hand, and so into the outside of the bottle.

The following experiments will confirm this account of the Leyden phial.

1. Place an electrified phial on wax; a small cork-ball held in your hand, suspended by a dry silk thread, and brought near to the wire, will first be attracted and then repelled: When in a repelled state, sink your hand, that the ball may be brought towards the outside of the bottle; it will be instantly attracted till it has parted with its fire.

If the outside of the bottle had a positive electrical atmosphere, as well as the inside and the wire, an electrified cork would be repelled from the one as well as the other.

2. From a bent wire sticking in the table, let a small linen thread hang down within half an inch of the electrified phial; touch the wire of the phial repeatedly with your finger; and, at every touch, you will see the thread instantly attracted by the outside of the bottle. As soon as you draw any fire from the inside by touching the wire, the outside draws in an equal quantity by the thread.

3. Fix a wire in the outside coating of the bottle, so as that bending upwards its ring-end may be level with the top or ring-end of the wire in the cork of the bottle, and at three or four inches distance. Then electrify the bottle, and place it on wax. If a cork, suspended by a silk thread, hang between these two wires, it will play incessantly from the one to the other, till the equilibrium between the inside and the outside of the bottle is restored.

4. Place a man on a cake of wax, and present him the wire of the electrified phial to touch, you standing on the floor and holding it in your hand. As often as he touches it, he will be electrified plus; and any one standing on the floor may draw a spark from him. The fire, in this experiment, passes out of the wire into him; and, at the same time, out of your hand into the outside of the bottle. Give him the electrical phial to hold, and touch the wire; as often as you touch it, he will be electrified minus, and may draw a spark from any one standing in the floor. The fire in this case passes from the wire to you, and from him into the outside of the bottle.

5. Lay two books, or two glasses, back to back, two or three inches distant, place the electrified phial on one of them, and then touch the wire; that book will be electrified minus, the electrical fire being drawn out of it by the outside of the bottle. Take off the bottle, and, holding it in your hand, touch the other with the wire; that book will be electrified plus, the fire passing into it from the wire, and the outside of the bottle is at the same time supplied from your hand.

The same explosion and shock happens, if the electrified phial is held in one hand by the hook of the wire, and the coating touched with the other, as when held by the coating and touched at the hook. To take the charged phial safely by the hook, and not at the same time diminish its force; it must first be set down on a non-conductor. The phial will be electrified as strongly, if held by the hook, and the coating applied to the globe or tube, tube, as when held by the coating and the hook applied; but the direction of the electrical fire, being different in the charging, will also be different in the explosion; the bottle charged through the hook will be discharged thro' the hook; the bottle charged thro' the coating will be discharged thro' the coating; because the fire must come out the same way it went in.

6. To prove this, take two bottles that were equally charged thro' the hooks, one in each hand; bring their hooks near each other, and no spark or shock will follow; because each hook is disposed to give fire, and neither to receive it. Set one of the bottles on glass, take it up by the hook, and apply its coating to the hook of the other; then there will be an explosion and shock, and both bottles will be discharged. [N.B. To charge a bottle commodiously thro' the coating; place it on a glass-stand; form a communication from the prime conductor to the coating, and another from the hook to the wall or floor; when it is charged, remove the latter communication before you take hold of the bottle, otherwise great part of the fire will escape by it.]

When the terms of charging or discharging the phial are used, it is in compliance with custom, and for want of better ones; since there is really no more electrical fire in the phial after what is called its charging than before, nor less after its discharging. Besides, the phial will not suffer what is called a charging, unless as much fire can go out of it one way as is thrown in by another. A phial cannot be charged standing on wax or glass, or hanging on the prime conductor, unless a communication be formed between its coating and the floor. But suspend two or more phials on the prime conductor, one hanging to the tail of the other, and a wire from the last to the floor, an equal number of turns of the wheel will charge them all equally, and each as strongly as a single one would have been.

When a bottle is charged in the common way, its inside and outside surfaces stand ready, the one to give fire by the hook, the other to receive it by the coating: yet as the first will not give out, unless the other can at the same instant receive in; so neither will the latter receive in, unless the first can at the same instant give out. When both can be done at once, it is done with inconceivable quickness and violence.

Glass has within its substance the same quantity of electrical fire at all times, and that quantity is very great in proportion to the mass of glass. This quantity it obstinately retains; and will have neither more nor less, though it will allow a change to be made in its parts and situation; that is, we may take away part from one of the sides, provided we throw an equal quantity into the other. Yet when the situation of the electrical fire is thus altered in the glass, it will not be altered, or in its natural state, till it be restored to its original equality; and this restitution cannot be made through the substance of the glass, but must be done by a conducting communication formed without from surface to surface. Thus the whole force of the bottle, and power of giving a shock, resides in the Glass itself; the coatings, or conducting substances in contact with the two surfaces, serving only to give and receive to and from the several parts of the glass; that is, to give in one side, and take away from the other. This was discovered by Dr Franklin, and proved by the following experiment: Purposing, (says he,) to analyze the electrified bottle, in order to find wherein its strength lay, we placed it on glass; and drew out the cork and wire, which for that purpose had been loosely put in. Then taking the bottle in one hand, and bringing a finger of the other near its mouth, a strong spark came from the water, and the shock was as violent as if the wire had remained in it, which shewed that the force did not lie in the wire. Then to find if it resided in the water, being crowded into and condensed in it, as confined by the glass, which had been our former opinion, we electrified the bottle again, and placing it on glass drew out the wire and cork as before; then taking up the bottle, we decanted all its water into an empty bottle, which likewise stood on glass; and taking up that other bottle, we expected, if the force resided in the water, to find a shock from it; but there was none. We judged then that it must either be lost in decanting, or remain in the first bottle. The latter we found to be true; for that bottle on trial gave the shock, though filled up as it stood with fresh unelectrified water from a tea-pot. ——To find, then, whether glass had this property merely as glass, or whether the form contributed any thing to it; we took a pane of sash-glass, and laying it on the hand, placed a plate of lead on its upper surface; then electrified that plate, and bringing a finger to it, there was a spark and shock. We then took two plates of lead of equal dimensions, but less than the glass by two inches every way, and electrified the glass between them, by electrifying the uppermost lead; then separated the glass from the lead; in doing which, what little fire might be in the lead was taken out, and the glass being touched in the electrified parts with a finger, afforded only very small pricking sparks, but a great number of them might be taken from different places. Then dexterously placing it again between the leaden plates, and compleating a circle between the two surfaces, a violent shock ensued. ——Which demonstrated the power to reside in glass as glass; and that the non-electrics in contact served only, like the armature of a loadstone, to unite the force of the several parts, and bring them at once to any point desired: it being the property of a non-electric, that the whole body instantly receives or gives what electrical fire is given to or taken from any one of its parts.

It is amazing to observe in how small a portion of glass a great electrical force may lie. A thin glass bubble about an inch diameter, weighing only six grains, being half filled with water, partly gilt on the outside, and furnished with a wire hook, gives, when electrified, as great a shock as a man can well bear. As the glass is thickest near the orifice, I suppose the lower half, which being gilt was electrified and gave the shock, did not exceed two grains; for it appeared, when broke, much thinner than the upper half. ——If one of these thin bottles be electrified by the coating, and the spark taken taken out through the gilding, it will break the glass inwards, at the same time that it breaks the gilding outwards. And since there is no more electrical fire in a bottle after charging than before, how great must be the quantity in this small portion of glass! It seems as if it were of its very substance and essence. Perhaps if that due quantity of electrical fire to ultimately retained by glass, could be separated from it, it would no longer be glass; it might lose its transparency, or its brittleness, or its elasticity.—Experiments may possibly be invented hereafter to discover this.

Of the Similarity between Lightning and Electricity.

1. Flashes of lightning are generally seen crooked, and waving in the air. The electric spark has always the same direction when it is drawn from an irregular body at some distance, or through a space in which the best conductors are disposed in an irregular manner, which is always the case in the heterogeneous atmosphere of our globe.

2. Lightning strikes the highest and most pointed objects in its way preferable to others, as high hills, and trees, towers, spires, masts of ships, points of spears, &c. In like manner, all pointed conductors receive or throw off the electric fluid more readily than those which are terminated by flat surfaces.

3. Lightning is observed to take the readiest and best conductor. So does electricity in the discharge of the Leyden phial. For this reason, it would be safer, during a thunder-storm, to have one's clothes wet than dry, as the lightning might then, in a great measure, be transmitted to the ground, by the water, on the outside of the body. It is found, that a wet rat cannot be killed by the explosion of the electrical bottle, but that a dry rat may.

4. Lightning burns. So does electricity. It will kindle hard dry rosin, spirits unwarmed, and even wood. It will fire gunpowder, by only ramming it hard in a cartridge, into each end of which pointed wires are introduced, and brought within half an inch of one another, and discharging a shock through them.

5. Lightning sometimes dissolves metals. So does electricity. The method in which Dr Franklin made electricity melt metals, was by putting thin pieces of them between two panes of glass, bound fast together, and sending an electric shock through them. Sometimes the pieces of glass, by which they were confined, would be shattered to pieces by the discharge, and be broken into a kind of coarse sand, which once happened with pieces of thick looking-glass; but if they remained whole, the pieces of metal would be missing in several places where it had lain between them, and instead of it a metallic stain would be seen on both the glasses, the stains on the under and upper glass being exactly similar in the minutest stroke.

6. Lightning rends some bodies. So does electricity. The electric spark will strike a hole through a quire of paper.—When wood, bricks, stone, &c. are rent by lightning, the splinters will fly off on that side where there is the least resistance. In like manner, when a hole is struck through a piece of pasteboard by an electrified jar, if the surfaces of the pasteboard are not confined and compressed, there will be a bur raised all round the hole on both sides of the pasteboard; but if one side be confined, so that the bur cannot be raised on that side, it will all be raised on the other side, which waysoever the fluid was directed. For the bur round the outside of the hole is the effect of the explosion, which is made every way from the center of the electric stream, and not an effect of its direction.

7. Lightning has often been known to strike people blind. And a pigeon, after a violent shock of electricity, by which it was intended to be killed, was struck blind likewise.

8. In a thunder-storm at Streatham, described by Dr Miles, the lightning stripped off some paint which had covered a gilded moulding of a panel of wainscot, without hurting the rest of the paint. Dr Franklin imitated this, by patting a slip of paper over the filleting of gold on the cover of a book, and sending an electric spark through it. The paper was torn off from end to end, with such force, that it was broken in several places; and in others there was brought away part of the grain of the Turkey leather in which the book was bound. This convinced the doctor, that if it had been paint, it would have been stripped off in the same manner with that on the wainscot at Streatham.

9. Lightning destroys animal-life. Animals have likewise been killed by the shock of electricity. The largest animals which Dr Franklin and his friends had been able to kill were a hen, and a turkey which weighed about ten pounds.

10. Magnets have been observed to lose their virtue, or to have their poles reversed, by lightning. Dr Franklin did the same by electricity. By electricity he frequently gave polarity to needles, and reversed them at pleasure. A shock from four large jars, sent through a fine sewing needle, gave it polarity, so that it would traverse when laid on water. What is most remarkable in these electrical experiments upon magnets is, that if the needle, when it was struck, lay east and west, the end which was entered by the electric blast pointed north; but that if it lay north and south, the end which lay towards the north would continue to point north, whether the fire entered at that end or the contrary. He also observed, that the polarity was strongest when the needle was struck lying north and south, and weakest when it lay east and west. He takes notice, that, in these experiments, the needle, in some cases, would be finely blue or, like the spring of a watch, by the electric flame; in which case the colour given by a flash from two jars only might be wiped off, but that a flash from four jars fixed it, and frequently melted the needles. The jars which the doctor used held seven or eight gallons, and were coated and lined with tinfoil.

To demonstrate, in the completest manner possible, the sameness of the electric fluid with the matter of lightning, Dr Franklin contrived to bring lightning from the heavens, by means of an electrical kite, which he raised when a storm of thunder was perceived to be coming on. This kite kite had a pointed wire fixed upon it, by which it drew the lightning from the clouds. This lightning descended by the hempen string, and was received by a key tied to the extremity of it; that part of the string which was held in the hand being of silk, that the electric virtue might stop when it came to the key. He found that the string would conduct electricity even when nearly dry, but that when it was wet it would conduct it quite freely; so that it would stream out plentifully from the key at the approach of a person's finger.

At this key he charged phials, and from electric fire thus obtained he kindled spirits, and performed all other electrical experiments which are usually exhibited by an excited globe or tube.

The first appearance of a thunder-storm (which generally happens when there is little or no wind) is one dense cloud, or more, increasing very fast in size, and rising into the higher regions of the air. The lower surface is black, and nearly level; but the upper finely arched, and well defined. Many of these clouds often seem piled one upon another, all arched in the same manner; but they keep continually uniting, swelling, and extending their arches.

At the time of the rising of this cloud, the atmosphere is generally full of a great number of separate clouds, motionless, and of odd and whimsical shapes. All these, upon the appearance of the thunder-cloud, draw towards it, and become more uniform in their shapes as they approach; till, coming very near the thunder-cloud, their limbs mutually stretch towards one another; they immediately coalesce, and together make one uniform mass. These are called adscititious clouds, from their coming in, to enlarge the size of the thunder-cloud. But, sometimes the thunder-cloud will swell, and increase very fast without the conjunction of any adscititious clouds, the vapours in the atmosphere forming themselves into clouds wherever it passes. Some of the adscititious clouds appear like white fringes, at the skirts of the thunder-cloud, or under the body of it; but they keep continually growing darker and darker, as they approach to unite with it.

When the thunder-cloud is grown to a great size, its lower surface is often ragged, particular parts being detached towards the earth, but still connected with the rest. Sometimes the lower surface swells into various large protuberances, bending uniformly towards the earth. And sometimes one whole side of the cloud will have an inclination to the earth, and the extremity of it will nearly touch the earth. When the eye is under the thunder-cloud, after it is grown large, and well formed, it is seen to sink lower, and to darken prodigiously; at the same time that a number of small adscititious clouds (the origin of which can never be perceived) are seen in a rapid motion, driving about in very uncertain directions under it. While these clouds are agitated with the most rapid motions, the rain generally falls in the greatest plenty; and if the agitation be exceeding great, it commonly hails.

When the thunder-cloud is swelling, and extending its branches over a large tract of country, the lightning is seen to dart from one part of it to another, and often to illuminate its whole mass. When the cloud has acquired a sufficient extent, the lightning strikes between the cloud and the earth, in two opposite places, the path of the lightning lying through the whole body of the cloud and its branches. The longer this lightning continues, the rarer does the cloud grow, and the less dark is its appearance; till, at length, it breaks in different places, and shows a clear sky. When the thunder-cloud is thus dispersed, those parts which occupy the upper regions of the atmosphere are equally spread, and very thin; and those that are underneath are black, but thin too; and they vanish gradually, without being driven away with any wind.

That thunder-clouds were sometimes in a positive as well as negative state of electricity, Signior Beccaria had discovered, before he heard of its having been observed by Dr Franklin or any other person. The same cloud, in passing over his observatory, electrified his apparatus sometimes positively, and sometimes negatively. The electricity continued longer of the same kind, in proportion as the thunder-cloud was simple, and uniform in its direction; but when the lightning changed its place, there commonly happened a change in the electricity of his apparatus. It would change suddenly after a very violent flash of lightning, but the change would be gradual when the lightning was moderate, and the progress of the thunder-cloud slow.

It was an immediate inference from his observations of the lightning abroad, and his apparatus within, that the quantity of electric matter, in an usual storm of thunder, is almost inconceivably great; considering how many pointed bodies, as trees, spires, &c. are perpetually drawing it off, and what a prodigious quantity is repeatedly discharged to or from the earth.

Considering the vast quantity of electric fire that appears in the most simple thunder-storms, he thinks it impossible that any cloud, or number of clouds, should ever contain it all, so as either to discharge or receive it. Besides, during the progress and increase of the storm, though the lightning frequently struck to the earth, the same clouds were the next moment ready to make a still greater discharge, and his apparatus continued to be as much affected as ever. The clouds must, consequently, have received at one place, the moment that a discharge was made from them in another. In many cases, the electricity of his apparatus, and consequently of the clouds, would instantly change from one kind to another several times; an effect which cannot be accounted for by any simple discharge or recruit. Both must have taken place in a very quick succession.

The extent of the clouds doth not lessen this difficulty: for, be it ever so great, still the quantity ought to be lessened by every discharge: and besides, the points by which the silent discharges are made are in proportion to the extent of the clouds. Nor is the difficulty lessened by supposing that fresh clouds bring recruits; for besides that the clouds are not ripe for the principal storm, till all the clouds, to a great distance, have actually coalesced, and formed one uniform mass, those recruits bear no sort of proportion to the discharge, and whatever it was, it would soon be exhausted. The fact, therefore, must be, that the electric matter is continually darting from the clouds in one place, at the same time that it is discharged from the earth in another. And it is a necessary consequence from the whole, that the clouds serve as conductors to convey the electric fluid from those places of the earth which are overloaded with it, to those which are exhausted of it.

That great quantities of electric matter do sometimes rush out of particular parts of the earth, and rise through the air into the higher regions of the atmosphere, he thinks is evident from the great quantities of sand, ashes, and other light substances, which have often been carried up into the air, and scattered uniformly over a large tract of country. No other known efficient cause of this phenomenon can be assigned, except the wind; and it has been observed when there was no wind stirring; and the light bodies have even been carried against the wind. He supposes, therefore, that these light bodies are raised by a large quantity of electric matter, issuing out of the earth, where it was overcharged with it, and attracting and carrying with it every substance that could serve as a conductor in its passage. All these bodies, being possessed of an equal quantity of the electric fluid, will be dispersed equally in the air, and consequently over that part of the earth where the fluid was wanting, and whither they serve to convey it. Had these bodies been raised by the wind, they would have been dispersed at random, and in heaps.

This comparatively rare phenomenon, he thinks, exhibits both a perfect image, and demonstration, of the manner in which the vapours of the atmosphere are raised to form thunder-clouds. The same electric matter, wherever it issues, attracts to it, and carries up into the higher regions of the air, the watery particles that are dispersed in the atmosphere. The electric matter ascends to the higher regions of the atmosphere, being solicited by the less resistance it finds there than in the common mists of the earth; which, at those times, is generally very dry, and consequently highly electric. The uniformity with which thunder-clouds spread themselves, and swell into arches, must be owing to their being affected by some cause which, like the electric matter, diffuses itself uniformly wherever it acts, and to the resistance they meet with in ascending through the air. As a proof of this, steam, rising from an electrified eolipile, diffuses itself with the same uniformity, and in similar arches, extending itself towards any conducting substance.

The same cause which first raised a cloud, from vapours dispersed in the atmosphere, draws it to those that are already formed, and continues to form new ones; till the whole collected mass extends so far, as to reach a part of the earth where there is a deficiency of the electric fluid. Thither too, will those clouds, replete with electricity, be strongly attracted, and there will the electric matter discharge itself upon the earth. A channel of communication being, in this manner, found, a fresh supply of electric matter will be raised from the overloaded part, and will continue to be conveyed by the medium of the clouds, till the equilibrium of the fluid between the two places of the earth be restored. When the clouds are attracted in their passage by those parts of the earth where there is a deficiency of the fluid, those detached fragments are formed, and also those uniform depending protuberances, which, in some cases, are the cause of water-spouts, and hurricanes.

That the electric matter, which forms and animates the thunder-clouds, issues from places far below the surface of the earth; and that it buries itself there, is probable from the deep holes that have, in many places, been made by lightning. Flashes of lightning have, also, been seen to arise from subterraneous cavities, and from wells. Violent inundations have accompanied thunderstorms, not occasioned by rain, but by water bursting from the bowels of the earth, from which it must have been dislodged by some internal concussion. Deep wells have been known to fill faster in thunder-storms, and others have constantly grown turbid at the approach of thunder.

This very rise, as well as the whole progress of thunder-clouds, has sometimes been in a manner visible. Exhalations have been frequently seen to rise from particular caverns, attended with a rumbling noise, and to ascend into the higher regions of the air, with all the phenomena of thunder-storms described above, according to the description of persons who lived long before the connection between electricity and lightning was suspected.

The greatest difficulty attending this theory of the origin of thunder-storms relates to the collection and insulation of electric matter within the body of the earth. With respect to the former, he has nothing particular to say. Some operations in nature are certainly attended with a loss of the equilibrium in the electric fluid, but no person has yet assigned a more probable cause of the redundancy of electric matter which, in fact, often abounds in the clouds, than what we may suppose possible to take place in the bowels of the earth. And supposing the loss of the equilibrium possible, the same cause that produced the effect would prevent the restoring of it; so that not being able to force a way, at least one sufficiently ready, through the body of the earth, it would issue at the most convenient vent into the higher regions of the air, as the better passage. His electrical apparatus, though communicating with the earth, has frequently, in violent thunder-storms, given evident sparks to his finger.

In the enumeration of the effects of thunder-storms, he observes that a wind always blows from the place from which the thunder-cloud proceeds; that this is agreeable to the observations of all mariners, and that the wind is more or less violent in proportion to the suddenness of the appearance of the thunder-cloud, the rapidity of its expansion, and the velocity with which the adhesive clouds join it. The sudden condensation of such a prodigious quantity of vapours must displace the air, and repel it on all sides.

He, in some measure, imitated even this effect of thunder, at least produced a circulation of all the air in his room, by the continued electrification of his chain.

Among other effects of lightning, he mentions the case of a man rendered exceeding stiff, presently after he was struck dead in a storm of thunder. But the most remarkable circumstance, in this case, was the lightning (chu- This matter of lightning, or of electricity, is an extreme subtle fluid, penetrating other bodies, and subsisting in them equally diffused.

When by any operation of art or nature, there happens to be a greater proportion of this fluid in one body than in another, the body which has most will communicate to that which has least, till the proportion becomes equal; provided the distance between them be not too great; or, if it is too great, till there be proper conductors to convey it from one to the other.

If the communication be through the air without any conductor, a bright light is seen between the bodies, and a sound is heard. In our small experiments we call this light and sound the electric spark and snap; but in the great operations of nature, the light is what we call lightning, and the sound (produced at the same time, though generally arriving later at our ears than the light does to our eyes) is, with its echoes, called thunder.

If the communication of this fluid is by a conductor, it may be without either light or sound, the subtle fluid passing in the substance of the conductor.

If the conductor be good and of sufficient bigness, the fluid passes through it without hurting it. If otherwise, it is damaged or destroyed.

All metals, and water, are good conductors.—Other bodies may become conductors by having some quantity of water in them, as wood, and other materials used in building, but not having much water in them, they are not good conductors, and therefore are often damaged in the operation by lightning.

Glass, wax, silk, wool, hair, feathers, and even wood, perfectly dry, are non-conductors: that is, they resist instead of facilitating the passage of this subtle fluid.

When this fluid has an opportunity of passing through two conductors, one good and sufficient, as of metal, the other not so good, it passes in the best, and will follow it in any direction.

The distance at which a body charged with this fluid will discharge itself suddenly, striking through the air into another body that is not charged, or not so highly charged, is different according to the quantity of the fluid, the dimensions and form of the bodies themselves, and the state of the air between them.—This distance, whatever it happens to be between any two bodies, is called their striking distance, as till they come within that distance of each other, no stroke will be made.

The clouds have often more of this fluid in proportion than the earth; in which case as soon as they come near enough (that is, within the striking distance) or meet with a conductor, the fluid quits them and strikes into the earth. A cloud fully charged with this fluid, if so high as to be beyond the striking distance from the earth, passes quietly without making any noise or giving light; unless it meets with other clouds that have less.

Tall trees, and lofty buildings, as the towers and spires of churches, become sometimes conductors between the clouds and the earth; but not being good ones, that is, not conveying the fluid freely, they are often damaged.

Buildings that have their roofs covered with lead, or other metal, and spouts of metal continued from the roof into in to the ground to carry off the water, are never hurt by lightning, as whenever it falls on such a building, it passes in the metals and not in the walls.

When other buildings happen to be within the striking distance from such clouds, the fluid passes in the walls, whether of wood, brick or stone, quitting the walls only when it can find better conductors near them, as metal rods, bolts, and hinges of windows or doors, gilding on wainscot, or frames of pictures; the silvering on the backs of looking-glasses; the wires for bells; and the bodies of animals, as containing watery fluids. And in passing through the house it follows the direction of these conductors, taking as many in its way as can assist it in its passage, whether in a straight or crooked line, leaping from one to the other, if not far distant from each other, only rending the wall in the spaces where these partial good conductors are too distant from each other.

An iron rod being placed on the outside of a building, from the highest part continued down into the moist earth, in any direction straight or crooked, following the form of the roof or other parts of the building, will receive the lightning at its upper end, attracting it so as to prevent its striking any other part; and, affording it a good conveyance into the earth, will prevent its damaging any part of the building.

A small quantity of metal is found able to conduct a great quantity of this fluid. A wire no bigger than a goose quill has been known to conduct (with safety to the building as far as the wire was continued) a quantity of lightning that did prodigious damage both above and below it; and probably larger rods are not necessary, though it is common to make them of half an inch, some of three quarters, or an inch diameter.

The rod may be fastened to the wall, chimney, &c., with staples of iron.—The lightning will not leave the rod (a good conductor) to pass into the wall (a bad conductor) through those staples.—It would rather, if any were in the wall, pass out of it into the rod to get more readily by that conductor into the earth.

If the building be very large and extensive, two or more rods may be placed at different parts, for greater security.

Small ragged parts of clouds suspended in the air between the great body of clouds and the earth (like leaf-gold in electrical experiments), often serve as partial conductors for the lightning, which proceeds from one of them to another, and by their help comes within the striking distance to the earth or a building. It therefore strikes through those conductors a building that would otherwise be out of the striking distance.

Long sharp points communicating with the earth, and presented to such parts of clouds, drawing silently from them the fluid they are charged with, they are then attracted to the cloud, and may leave the distance so great as to be beyond the reach of striking.

It is therefore that we elevate the upper end of the rod six or eight feet above the highest part of the building, tapering it gradually to a fine sharp point, which is sure to prevent its rusting.

Thus the pointed rod either prevents a stroke from the cloud, or, if a stroke is made, conducts it to the earth with safety to the building.

The lower end of the rod should enter the earth so deep as to come at the moist part, perhaps two or three feet; and if bent when under the surface so as to go in a horizontal line six or eight feet from the wall, and then bent again downwards three or four feet, it will prevent damage to any of the stones of the foundation.

A person apprehensive of danger from lightning, happening during the time of thunder to be in a house not secured, will do well to avoid sitting near the chimney, near a looking glass, or any gilt pictures or wainscot; the safest place is in the middle of the room, (so it be not under a metal lustre suspended by a chain), sitting in one chair and laying the feet up in another. It is still safer to bring two or three matresses or beds into the middle of the room, and folding them up double, place the chair upon them; for they not being so good conductors as the walls, the lightning will not choose an interrupted course through the air of the room and the bedding, when it can go thro' a continued better conductor, the wall. But where it can be had, a hammock or swinging bed, suspended by silk cords equally distant from the walls on every side, and from the ceiling and floor above and below, affords the safest situation a person can have in any room whatever; and what indeed may be deemed quite free from danger of any stroke by lightning.

In order to secure ships from sustaining damage by lightning, a copper road, about the thickness of a goose quill, should be connected with the spindles and iron work of the masts continued down to the deck, and from thence, in the most convenient direction, till the end of the rod be always in contact with the sea-water.

With regard to powder-mills and magazines, the apparatus to conduct the lightning from them should be detached from the buildings themselves, and conveyed to the nearest water.

Of Medical Electricity.

The first application of electricity to the cure of diseases was made by M. Jallabert, professor of philosophy at Geneva, on a locksmith whose right arm had been paralytic fifteen years. He was brought to M. Jallabert on the 26th of December 1747, and was completely cured by the 28th of February 1748. In this interval he was frequently electrified, sparks being taken from the arm, and sometimes the electrical shock sent through it.

The report of this cure at Geneva, engaged Mr Sauvages of the academy in Montpellier to attempt the cure of paralytics, in which he had considerable success.

In the year 1757, Mr Patrick Bryden, in a few days, performed a compleat cure of a hemiplegia, and indeed an almost universal paralytic affection of two years continuance.

Dr Hart, Dr Wilson, Mr Lovet, Mr Wesley, and many others, relate a number of cases wherein the palsy was either cured or mitigated by electricity.

Dr Watson cured an universal tetanus, in the year 1762, by electrifying the patient, at proper intervals, for three months. Dr Franklin and others mention some paralytic cases, in which electricity seemed rather to make the patient worse than better.

Mr Wilson cured a woman of deafness of seventeen years standing.—And Mr Lover considers electricity as a specific in all cases of violent pains, obstinate headaches, the sciatica, and the cramp. The toothache, he says, is generally cured by it in an instant. He relates a case from Mr Floyer surgeon at Dorchester, of a compleat cure of a gutta serena; and another of obstinate obstructions in two young women.

De Haen says, that he never failed to cure St Vitus's dance by electricity; and found it of use in some cases of deafness.

Hitherto electricity has been generally applied to the human body either in the method of drawing sparks, as it is called, or of giving shocks. But these operations are both violent, and though the strong concussion may suit some cases, it may be of difference in others, where a moderate simple electrification might have been of use.

The great objection to this method is the tediousness and expense of the application. But an electrical machine might be contrived to go by wind or water, and a convenient room might be annexed to it; in which a floor might be raised upon electrics, a person might sit down, read, sleep, or even walk about during the electrification. It were to be wished, that some physician of understanding and spirit would provide himself with such a machine and room. No harm could possibly be apprehended from electricity, applied in this gentle and insensible manner, and good effects are at least possible, if not highly probable.