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centres, when the cone afd has made one revolution, the cone ade will have made but half a revolution, and every, part in each cone, equally diftant from the centre a, will have the fame proportion in their revolutions to each other, as f 1, f 2, f 3, &c. will have made two revolutions to the points e 1, e 2, e 3, &c. for one revolution of the other cone respectively, &c. Now, if the cones are fluted, or have teeth cut in them, diverging from the centre a to the bases de, df (fig. 16), they would then become bevel geer. The teeth at the point of the cone being fmall, and of little ufe, may be cut off; or, instead of the two cones, may be used two fhafts, with bevel wheels fixed to them, as the fhaft ab (fig. 18), with the bevel wheel cd, which turns the bevel wheel ef, with its fhaft big and the teeth work freely into each other, as in figure 16. The teeth may be made of any dimenfions, according to the ftrength required, and by this means a motion may be communicated in any direction, or to any part of a building, with very little trouble and friction.

The method of contructing the wheels for any proportion, is as follows:-Draw the line ab (fig. 21) to represent a fhaft of a wheel; draw the line ed to interfect the line ab, in the direction that the motion is to be conveyed, and the line ed will represent the other shaft of the motion.

Then fuppofe the fhaft ed is to revolve three times in the time that the shaft ab revolves once; draw the parallel line ii at any distance, from a fcale (fuppofe one foot); then draw the other parallel line k k at three feet distance; after which, draw the line wx through the interfections of the two fhafts ab and ed, and likewife through the intersections of the two parallel lines ii and kk, in the points xy, which will be the pitch line of the two bevel wheels, or the lines where the teeth of the two wheels act on each other, as may be feen in figure 19, where there are three wheels.

Where it is required to communicate a continued uniform motion, and where the angle does not exceed 40 degrees,

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and also 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 fhewn 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 shafts 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 refiftance.

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 elaftic fubtle fluid, called by philofophers, the electric fluid; and when any body pof

feffes

feffes more or lefs of this fluid than what naturally belongs to it, feveral effects are visible in it, and the body is said 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 surfaces; but there are many bodies which will not fuffer this fluid to pafs through them, while others freely permit it. Those bodies through which the electric fluid can pass are called conductors of electricity, of which the most perfect are metals of all kinds. And thofe bodies through which the electric fluid cannot pass 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 substances 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 pafs, or non-conductors, it must remain overloaded; or if it have loft part of its natural share of electric matter, it must remain exhausted; because the bodies which furround it prevent any of the electric fluid from entering or coming out of it, and the body is then faid to be infulated.

There are two principal theories of electricity, each of which has had its advocates. The one is, that of two diftinct electric fluids, repulfive with refpect 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 these two fluids are equally attracted by all bodies, and exist in intimate union in their pores; and in this state they fhew no mark of their existence.

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