WEIGHING-MACHINE is any contrivance by which the weight of an object may be ascertained. Under BALILNCE the principles of the machines by which materials of comparatively small magnitude are weighed are explained. The scales with equal arms and equipoised weights require no further elucidation ; but a short notice may be given of some others. The steelyard is a kind of balance or weighing machine, consisting of a lover of unequal arms. The most common kind of steelyard, which is often called the Roman balance, is a lever of the first order, and is used by suspending the article to be weighed from the end of the shorter arm, and sliding a determinate weight along the longer arm, to a greater or less distance from the fulcrum, until the instrument remains in equilibrium in an horizontal position ; the weight of the substance attached to the short arm of the lever being indicated by observing the position of the moveable balance weight with respect to a graduated scale marked upon the long arm of the steelyard. In the common steelyard a hook or hooks are usually suspended from the short arm, to hold the article the weight of which is to be ascertained ; but sometimes a scale-plate or dish suspended by chains is added. The moveable weight is commonly attached to a ring, the form of which enables It to rest in notches cut on the upper edge of the atee13-ard, corresponding with the graduations engraved on its aide. A ring or hook is also attached to the fulcrum, so that the instrument may be couvoniently hung upon • fixed support, or If small, held in the hand ; and • vertical index or pointer, smiler to that attached to the beam cf common scales, is sometimes added. The fulcrum, and the axis from which the weight is suspended, should, when much nicety le required, be provided with knife edges or boar ings resembling those used in other lever-balances. litany steelyards are supplied with a second fulcrum ; the two being placed at different distances from the point to which the hook or scale is attached, and having their respective pointers and suspending-hooks on opposite sides of the lever, or rather, when held in the position for use, one above and the other below it, as shown in the following cut of an ancient Roman steelyard. In using a steelyard of this kind, capable of weigh ing from one to sixty pounds, the fulcrum which is nearest to the middle is used if the article be under fifteen pounds ; while if it exceed that weight, the instrument must be inverted, and suspended from the fulcrum which divides the lever most unequally.
Several ingenious bent-lover balances have been contrived, some of which, from the circumstance of the levers being of unequal arms, resemble the steelyard in principle; and various modifications of the steelyard have been invented for delicate scientific purposes, or for adapting it to the purpose of weighing very heavy bodies. Of the latter class is that which is employed usually at the toll-gates on roads for the purpose of determining the weights of laden carriages. In order to prevent the roads from being too much cut up, the burdens allowed to be conveyed along them by carts or waggons are made to depend on the breadth of the wheels; and a fine is imposed for any excess above the regulated quantity.
Tho usual weighing-machine may be described, in a general way, as • platform sunk on a level with the road, and made to rest at four points on a double lever of the second kind. The extremities of the arms of those levers rest upon a third lever, which may be of the first or second kind; and this last lover may either serve as a steelyard, or may be connected with one arm of an ordinary balance, or with the extremity of a steelyard.
But to bo more particular, let ',nen be the plan of a rectangular pit sunk in the ground, from 8 to 12 feet long, 6 feet broad, and about , 2 feet deep, the sides and bottom being lined with brickwork or iron ; and let St N 11 a represent a longitudinal section of the pit perpendicu larly to the ground : a b cd, b'e d' are two trapezoidal frames of iron, acting as levers; and each of the side bars, as ac or bd, hie, in the vertical position, the form which is represented by ac, in the sec tiou. At each extremity a, b, a', b', in the p the frames havea conical steel point, which appears at a, a' in the section ; this rests in a hemispherical cavity made in a die or cylinder of the same metal, which is either attached to the iron-work forming the sides of the pit, or is supported on a block of stone sunk in the ground at each of the four interior angles • and under the shorter aide, cd or c'd', of each frame there is a wedge-like prism of tempered steel, having its edge parallel to that side. The ends of these prisms appear at c and c' in the section.