# BALANCE

, one of the six simple powers in mechanics, chiefly used in determining the equality or difference of weight in heavy bodies, and consequently their masses or quantities of matter.

The balance is of two kinds, the ancient and modern. The ancient or Roman, called also Statera Romana, or Steelyard, consists of a lever or beam, moveable on a centre, and suspended near one of its extremities. The bodies to be weighed are suspended from the shorter end, and their weight is shewn by the division marked on the beam, where the power or constant weight, which is moveable along the lever, keeps the steelyard in equilibrio. This balance is still in common use for weighing heavy bodies.

The modern balance, now commonly used, consists of a lever or beam suspended exactly in the middle, and having scales suspended from the two extremities, to receive the weights to be weighed.

In either case the lever is called the jugum or the beam, and its two halves on each side the axis, the brachia or arms; also the line on which the beam turns, or which divides it in two, is called the axis; and when considered with regard to the length of the brachia, is esteemed only a point, and called the centre of the balance, or centre of motion: the extremities where the weights are applied, are the points of application or suspension; the handle by which the balance is held, or by which the whole apparatus is suspended, is called trutina; and the slender part perpendicular to the beam, by which is determined either the equilibrium or preponderancy of bodies, is called the tongue of the balance.

From these descriptions we easily gather the characteristic distinction between the Roman balance and the common one, viz, that in the Roman balance, there is one constant weight used as a counterpoise, the point where it is suspended being varied; but, on the contrary, in the common balance or scales, the points of suspension remain the same, and the counterpoise is varied. The principle of both of them may be easily understood from the general properties of the lever, and the following observations.

The beam ABC, the principal part of the balance, is a lever of the first kind; but instead of resting on a fulcrum, it is suspended by a handle, &c, fastened to its centre of motion B: and hence the mechanism of the balance depends on the same theorems as that of the lever. Consequently as the distance between the centre of motion and the place of the unknown weight, is to the distance between the same centre and the place of the known weight, so is the latter weight, to the former. So that the unknown weight is discovered by means | of the known one, and their distances from the common centre of motion; viz, if the distances from the centre be equal, then the two weights will be equal also, as in the common balance; but if the distances be unequal, then the weights will also be unequal, and in the very same proportion, alternately, the less weight having so much the greater distance, as in the steelyard. The Common Balance or Scales. The two brachia AB, BC, should be exactly equal in length, and in weight also when their scales D and E are fixed on their ends; the beam should hang exactly level or horizontal in the case of an equipoise; and for this purpose the centre of gravity of the whole should fall a little below the centre of motion, and but a little, that the balance be sufficiently sensible to the least variation of weight: the friction on the centre should also be as small as possible. The Steel Yard.

Having made a proper bar of steel AB, tapering at the longer end, and very strong at the other, suspend it by a centre C near the shorter or thicker end, so that it may exactly balance itself in equilibrio, and prepare a constant weight I to weigh with: then hang on any weight, as one pound for instance, at the shorter arm, and slide the constant weight backwards and forwards upon the longer arm, till it be just in equilibrio with the former; and there make a notch and number 1, for the place of 1 pound: take off the 1lb, and hang a two pound weight in its stead at the shorter arm; then slide the constant weight back on the longer arm, till the whole come again into equilibrio, making a notch at the place of the constant weight and the number 2, for the place of 2lb. Proceed in the same manner for all other weights 3, 4, 5, &c; as also for the intermediate halves and quarters, &c, if it be necessary; always sus- pending the variable weights at the end of the shorter arm, shisting the constant weight so as to balance them, and marking and numbering the places on the longer arm where the constant weight always makes a counterpoise. The use of the Steelyard is hence very evident: the thing whose weight is required being suspended by a hook at the short end, move the constant weight backwards and forwards on the longer arm, till the beam is balanced horizontally: then look what notch the constant weight is placed at, and its number will shew the weight of the body that was required. DC is the handle and tongue; F the centre of motion; EG a scale sometimes hung on at the end by the hook H. The Bent-Lever Balance.

This instrument operates by a fixed weight, C, increasing in power as it ascends along the are FG of a circle, and pointing by an index to the number or division of the are which denotes the weight of any body put into the scale at E. And thus one constant weight serves to weigh all others, by only varying the position of the arms of the balance, instead of varying the places or points of suspension in the arms themselves.

The Deceitful Balance. This operates in the same manner as the steelyard, and cheats or deceives by having one arm a little longer than the other; though the deception is not perceived, because the shorter arm is made somewhat heavier, so as to compensate for its shortness, by which means the beam of the balance, when no weights are in the scales, hangs horizontal in equilibrio. The consequence of this construction is, that any commodity put in the scale of the longer arm, requires a greater weight in the other scale to balance it; and so the body is fallaciously accounted heavier than it really is. But the trick will easily be detected by making the body and the weight change places, removing them to the opposite scales, when the weight will immediately be seen to preponderate.

Assay-Balance. This is a very nice balance, used in determining the exact weights of very small bodies. Its structure is but little different from the common sort; | except that it is made of the best and hardest steel, and made to turn with the smallest weight.

Hydrostatical Balance. This is un instrument for determining the specific gravity of bodies. See HYDROSTATICAL, and Specific Gravity.

Balance

, in Astronomy, the same as Libra.

Balance of a Clock or Watch, is that part which, by its motion, regulates and determines the beats. The circular part of it is called the rim, and its spindle the verge; there belong to it also two pallets or nuts, that play in the fangs of the crown-wheel: in pocket watches, that strong stud in which the lower pivot of the verge plays, and in the middle of which one pivot of the crown-wheel runs, is called the potence: the wrought piece which covers the balance, and in which the upper pivot of the balance plays, is the cock; and the small spring in the new pocket watches, is called the regulator.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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