and alfo where the equality of the motion is not regarded, the universal joint may be used (fig. 22) instead of the bevel geer. This joint may be constructed by a cross, as shown in the figure; or with four pins faftened at right angles upon the circumference of a hoop, or folid ball. This is of great ufe in fome machines, where the tumbling fhafts are continued to a great distance from the moving power, as it is in cotton-mills. The fhafts, by applying this joint, may also be cut to any length, which is a great advantage where there is much resistance. CHAP. XVI. OF ELECTRICITY. SECT. I. THE PRACTICAL PART OF ELECTRICITY. THE earth, air, and all terreftrial bodies are fuppofed to contain a certain quantity of an elastic subtle fluid, called by philofophers, the electric fluid; and when any body pof felfes feffes more or lefs of this fluid than what naturally belongs to it, feveral effects are vifible in it, and the body is faid to be electrified. This certain quantity of electric fluid found in all bodies could never be increased or diminished, if all bodies admitted the paffage of this electric fluid through their pores or along their furfaces; but there are many bodies which will not fuffer this fluid to pafs through them, while others freely permit it. Thofe bodies through which the electric fluid can pafs are called conductors of electricity, of which the most perfect are metals of all kinds. And those bodies through which the electric uid cannot pafs are called non-conductors of electricity, of which the most perfect are glass, refin, fealing-wax, fulphur, bees-wax, and baked wood, among folids; and oils and air, among fluids. But all substances become conductors when they are made very hot. Conducting fubftances are also called non-electrics, and non-conducting fubftances are called electrics. Into these two claffes all bodies are divided by electricians. When any body has acquired an additional quantity of electric matter, and is furrounded with other bodies through which the electric fluid cannot pass, or non-conductors, it must remain overloaded; or if it have loft part of its natural share of electric matter, it must remain exhaufted; because the bodies which furround it prevent any of the electric fluid from entering or coming out of it, and the body is then said to be infulated. There are two principal theories of electricity, each of which has had its advocates. The one is, that of two distinct electric fluids, repulfive with respect to themselves and attractive of one another, adopted by M. Du Fay, on discovering the two oppofite fpecies of electricity, viz. the vitreous and refinous, which is fince new-modelled by Mr. Symmer. Upon this hypothefis thefe two fluids are equally attracted by all bodies, and exift in intimate union in their pores; and in this ftate they flow no mark of their existence. But But the friction of an electric body against a rubber separates these fluids, and causes the vitreous electricity of the rubber to pafs to the electric, then to the prime conductor of the machine, while the refinous electricity of the conductor and electric is communicated to the rubber: thus the quality of the electric fluid poffeffed by the conductor and the rubber is changed, while the quantity remains the fame in each. In this feparated ftate the two electric fluids will exert their respective powers; and any number of bodies charged with either of these may repel each other, attract those bodies that have lefs of each particular fluid than themselves, and be still attracted more by bodies that are either only destitute of it, or loaded with the contrary. In this theory the electric spark makes a double current; one fluid paffing to the elec trified conductor from any fubftance prefented to it, while the fame quantity of the other fluid paffes from it; and when each body receives its natural quantity of both fluids, the balance of the two powers is restored, and both bodies are unelectrified. The other theory, and that which is commonly received, is that diftinguifhed by the name of pofitive and negative electricity, fuggefted by Dr. Watfon, and demonftrated by Dr. Franklin; in which it is fuppofed, that all bodies poffefs a certain fhare of one and the fame fluid, which is extremely fubtle and elaftic, by which the particles of it are ftrongly attracted, as they are repelled by one another. When bodies poffefs their natural fhare of this fluid, they are faid to be in an unelectrified ftate; but when the equilibrium is deftroyed, and they have an additional quantity from other bodies, or when they lofe part of their natural flare by the communication to other bodies, they then become electrified, and exhibit electrical appearances; which are generally the fame in both cafes. In the former cafe they are faid to be electrified pofitively, or plus; and in the latter cafe negatively, or minus. It is alfo fuppofed, that electrics always contain contain an equal quantity of this fluid; fo that there can be no increase on one fide without a proportional decrease or lofs on the other, and vice versa. And as the electric will not fuffer the fluid to pass through its pores, there will be an accumulation on one fide, and a correfponding deficiency on the other; then, connecting both fides together by proper conductors, the equilibrium will be reftored, by the rufhing in of the redundant fluid from the overcharged surface to the exhausted one. Thus, if an electric be rubbed by a conducting fubftance, the electricity is only conveyed from one to the other, the one giving what the other receives: and if one be electrified pofitively, the other will be electrified negatively, unless the lofs be fupplied by other bodies connected with it, as in the cafe of the electric and infulated rubber of a machine. Thus, bodies differently electrified will naturally attract each other, till they mutually give and receive an equal quantity of the electric fluid, and then the equilibrium between them will be restored. The method of disturbing the equilibrium of the electric fluid in bodies, or of making it pafs from one to another, is chiefly friction, or a flight rubbing of them one against the other; when the electric fluid will generally leave the rougher furface, and pass upon the fmoother; or it leaves the least perfect electric, and paffes to the more perfect one of the two. Thus, if a fimooth glafs tube (fig. 1, plate 20) be drawn through the hand, the effect of the friction makes the electric fluid leave the hand and pass to the glass tube, which is the more perfect electric of the two, where it will remain in addition to its natural quantity. For the electric fluid cannot poffibly leave the glafs, because neither the glafs nor the furrounding air are conductors of electricity; but if a conducting fubftance, as the finger, or a piece of metal, be presented to any part of the glass, the electric fluid will leave the glass and pass into them; and if the finger, or metal, be presented to every part of the tube fucceffively, the whole of the redundant fluid will leave the tube, and it will retain only its natural natural share. Here the glafs is faid to be excited, because the friction feems to excite the electric power which was in the glass. In the fame manner the friction of the glass globe against the rubber in the electrical machine makes the electrical fluid which was in the rubber pass to the glass, from whence it is conveyed to the prime conductor, the points of which are presented to every part of the globe in fucceffion, as it is turned in the machine; and as the friction is continued, there will be a conftant supply of electric fluid to the prime conductor (though other bodies be presented to it), and keep discharging all the while in visible sparks. The hand, in the former of these cafes, and the rubber, in the latter, part with their natural share of electric fluid to the glass against which they are rubbed, but receive an immediate supply from the conducting fubftances to which they are connected; and these are again supplied by the general mass of fluid that is in the earth. Again, if a stick of sealing-wax, a piece of fulphur, or a tube of rough glass, be drawn through the hand, the electric fluid belonging to them will pass from them to the hand, and being furrounded by the air, which is a non-conductor, they remain exhausted, and are ready to take sparks of electric fire from any bodies presented to them. The fulphur, fealing-wax, &c. in this case are said to be excited, as well as the glass, which was overloaded with fluid, though the state they are in be the reverse of one another. It is impoffible to distinguish by the eye the courfe of the electric matter, its velocity is so great. There are a variety of inventions for the conftruction of the electrical machine, but the most fimple is that reprefented in figure 2, which, by reason of its fimplicity, is not liable to be put out of order, as it has neither wheel nor ftring, though both might be attached thereto, if required. It may also be fixed firm on a table, and easily taken off : VOL. II. 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