MEASUREMENT OF SOLIDS.
The measurement of solids may be determined by two methods— namely, (T) by linear measure and (2) by gravity. As the appliances for the first method of measurement consist for the most part of simple instruments—for example, the foot-rule, tape-line, etc.—we shall confine the following treatment to those machines whose measurement is by grav ity or weight. Weight in experimental philosophy and in commerce is the measure of the force by which any body or a given portion of any substance gravitates to the earth. The process by which this measure is obtained is called " weighing," and when required, as in many philo sophical experiments, to be performed with great accuracy, is a tedious and delicate operation. The measurement of weight, like that of exten sion, consists in the comparison of the object to be measured with some conventional standard. But it is impracticable to fix such a standard by any written law or by oral description, for it is impossible to communicate by words, without reference to a perceptible object, any adequate idea of a or of a foot. Standards of linear measure not accurately defined, but having an average value sufficiently well known for the rude purposes of mankind in the early stages of civilization, were furnished by the different parts of the human body, from which are derived the foot, cubit, span, etc. (See p. 382.) A method of comparing the weights of bod ies does not suggest itself so readily to the mind as does a method of com paring their linear dimension. A balance is necessary, whose construction requires some degree of mechanical knowledge; hence the art of weighing, though of great antiquity, was probably practised at a later period and in a less accurate manner than was that of measuring. There have also been much less definiteness and a much greater variety in standards of weight than in those of measure, as will readily be understood if we consider the origin and import of such terms as stone, lead, etc. The term "pound" (pnehts) implies weight only indefinitely. As a unit of weight, the "grain " (granum), taken from the grains or corns of wheat, was per haps the only denomination of weight that would universally convey anything like a precise idea. The discovery of specific gravity by
Archimedes led to many important applications of measures of gravity.
Weighing Machines, or scales, measures, and weights, have in some form been in use from time immemorial. Pliny ascribes their invention to the Romans, but they were known and employed many centuries anterior to Roman history. In very ancient Egyptian paintings the merchant is seen with his scales carefully weighing his wares, and in China scales have been in use from the dawn of history.
original form of weighing-scales was probably a bar suspended at its centre, with a board or a shell suspended from each end, one to contain the weight and the other the article to be weighed. In early times, before the coinage of money, the precious metals were weighed out; the duty of weighing being regulated by the municipality and attended to by public weighers, as -is seen in the Egyptian monuments and as is recorded in classic literature. Figure 1 (pl. 121)is from an ancient Egyp tian papyrus in the British Museum, representing the "Ritual of the Dead." The Biblical balance was similar to that of the Egyptians; the ends were of equal length, and the beam was suspended at its mid-length. When Abraham (186° B. c.) bought from Ephron the Hittite the field containing the cave (Gen. xxiii. 13-16), he " weighed to Ephron . . . . four hundred shekels of silver current money with the merchant." The sale was made in the presence of witnesses, and is believed to be the ear liest transfer of land of which a record survives.
The balance of Archimedes was a beam with a bowl suspended from a fixed point at each end. On one arm of the beam, which was graduated from the fulcrum to the point of suspension of one of the bowls, was a movable weight; by adjustment on the arm the weight made a counter poise eqnal to the difference between the weights in the respective bowls.