The //bra or balance of the Romans, in its simplest form, consisted of a mere beam with a pair of scales,' one at each cud, and a ring or short chain placed in the centre of the beam to serve as a handle by which to poise it. In some cases the beam was furnished with an index or tongue working in an eye to mark the variations in weight; and sometimes the beam was divided into fractional parts with a weight attached to it, by means of which the difference in weight between two objects could be decided without the necessity of having recourse to a number of fractional weights for the purpose.
Balances for delicate operations, such as those used in assaying and in chemical manipulation, are made with extreme care. The sensitiveness of a balance so constructed may be carried to an almost inconceivable extent. Analytical balances are usually made to carry one thousand grains in each pan, and to " turn " with the part of a grain. There are in the English mint several large balances calculated to weigh from moo to 5000 ounces Troy, sonic of which will turn with one-tenth of a grain when loaded with woo ounces in each scale, or with part of the weight.
steelyard is a lunch later apparatus than the balance, and is supposed to be an invention of the Chinese. The frequent Biblical references to false and unequal balances show that the lever-balance on the principle of the steelyard was unknown. The Roman sialeree exhibited in Figure 2 (pl. 121), which are from originals discovered at Pompeii, arc the same in principle as the modern steelyard and weigher's beam. The sialera consists of the yard (maims), which is divided into fractional parts (buncia) and suspended from above by a hook or a chain. The short end of the yard is furnished with a hook, and sometimes with a scale, for affix ing or for holding the objects to be weighed, the longest end of the yard being provided with a sliding weight.
The lever of unequal lengths from the fulcrum or point of suspension affords a convenient mode for determining weights of various objects with but a single weight, the object being suspended from the end of the short arm, while the "bob" is shifted along the graduated longer arm until it forms an exact counterpoise. This is the modern steelyard, which was probably so called in England from its material and former length. The Merchants of the Steelyard were a company of foreigners, chiefly Flemish and Germans, in London (1252), who were long the only exporters of the staple commodities of England.
The on the principle of the steelyard has for many years been used in the United States for all purposes, from that of the letter scale (lig. 6), weighing half ounces, to that of the weigh-lock scales, whose weighing capacity is more than T,000,000 pounds.
Platform Lever-scales are those on which may be placed the object to be weighed. They have numerous forms and uses by which their construc tion is determined and their names designated. There are, for example, counter platform-scales 12r, fig. 7) of a comparatively small size; ware house scales (fig. 8) which are adapted for weighing boxes, barrels, sacks, and large packages, and which can be moved about the floor; also stock and hay scales (fig. 13), still larger in form, which are for weighing cattle or for weighing a wagon and its load. The principle of an ordinary plat
form-scale is readily understood: the graduated knife-edge beam is suitably supported at its fulcrum end, which is connected by a rod to the pivoted levers beneath the platform, and the outer end of the beam rests in a depending arm, so that the beam may be maintained in horizontal position and its vibration checked during the process of weighing. At each corner of the loose platform there is a downward-projecting steel-faced plate, which rests on the knife-edge of the levers, whose free ends are arrested in their downward movement by means of loops. From this description it will be seen that a package placed on the platform will depress the levers, which in turn bear on the connecting-rod attached to the fulcrum end of the beam. If now the weight or poise is slid along the beam until there is a true balance of the beam end, the point on the graduation of the beam at which this balance is effected will indicate the weight of the package. Connected to the outer end of the beam by means of a short rod is the suspended counterpoise. which serves also for increasing the weighing capacity of the machine by supporting slotted weights placed thereon.
Various kinds of weighing scales in common use are illustrated in Figures 3 to 15. Figure 5 represents a single-beam market-scale with sliding poise; Figure 7 is a grocer's simple lever-scale; Figure 4 is a compound-lever platform-scale for stores or for domestic purposes; Fig ure 3 is an even balance with weights, weighing from half ounces to six pounds; and Figure 9 is a hopper-scale for weighing grain at elevators, the hopper having a capacity of one thousand bushels. The grain is transferred from the railway car or from transports by means of bucket elevators to the top of the elevator building and discharged into the scale hopper. A slide in the bottom of the scale hopper admits of the contents of the box, after being weighed, being discharged into the grain-bins be neath. With such scales is used the combination beam shown in Figure II, which consists of a series of graduated beams adapted for weighing different kinds of grain—oats, wheat, etc. Figure r4 exhibits a coal dealer's scale, which, for determining the gross weight and the tare, fre quently has a compound beam similar in form to the compound beam for hay-scales shown in Figure 12, the upper beam of which records the weight in hundreds and thousands, the fractional parts of the weight being recorded on the lower beam. The stock scale (fig. r3) and railroad-track scale (jig. r5) are modifications of the coal-dealer's scale above described. Suspended scales are represented by the abattoir scale in Figure to, which is designed for weighing dressed beef (one quarter or more at a time), dead hogs, hogs heads of sugar, coal in buckets, etc., which are handled on a suspended track, a section of the track being cut out and hung on the scale levers which are placed above it. The goods are weighed as they pass over the track suspended to small carriages or wheels, as seen in the illustration. These scales are extensively employed in large refrigerators, slaughter houses, etc.