WEIGHT

In Physics, that property of bodies in virtue of which they tend towards the centre of the earth. In this sense, weight is synonymous with gravity. See Gravity. Weight, in Mechanics, denotes the resistance to be overcome by a machine, whether in raising, or sustaining, or moving a heavy body. The force applied to the machine for this purpose is called the moving power; and where the equilibrium subsists, the difference between the weight and the moving power is the purchase of the machine. In all cases of equilibrium by the intervention of machinery, if the machine be put in motion by a small additional force, the space pa ed over by the moving power will be greater than that pa ed over by the weight, in proportion as the weight is greater than the moving power; or the product of the weight by its velocity will be equal to the product of the moving power by its velocity. See Momentum, Virti;al Velocity. Weight, in Experimental Philosophy and Commerce, is the measure of the force by which any body, or a given portion of any substance, gravitates to the earth The proce by which this measure is obtained is callec weighing; and when required, as in many philosophical^ experiments, to be performed with great accuracy, is a tedious and delicate operation. See Balance. The measurement of weight, like that of extension, consists in the comparison of the thing to be measured with some conventional standard. But it is impo ible to fix such a standard by written law or oral description; for it is impo ible to communicate by words, and without reference to a sensible object, any adequate idea of a pound-weight, or footrule. Standards of linear measure, not accurately defined indeed, but having an average value sufficiently well known for the rude purposes of mankind in the early stages of society, were furnished by the different parts of the human body; hence the measures foot, cubit, span, pace, 8lc. (See Measures.) But the method of comparing the weights of bodies does not suggest itself so readily to the mind as the comparison of linear dimension, and is not so easily accomplished. A balance is nece ary, the construction of which requires some degree of' mechanical knowledge. Hence the art of weighing, though of great antiquity, (see Goguet, Origine des Lois, Sjc.) was probably practised at a later period, and in a still le accurate manner, than that of measuring. There has also been a much greater variety of standards of weight, still le definite than those of measure, as will be readily conceived by considering the origin and import of such terms as stone, load, last, . The term pound (pond us) implies only weight indefinitely. The grain, as a standard of small weight, being taken from the grains or corns of wheat, was perhaps the only denomination of weight that would universally convey any thing like a precise idea. As there is a constant ratio between the volumes and weights of the same substances when placed in the same physical circumstances, it is obvious that standards of weight may be derived from those of measure. For example, a cubic foot or a cubic inch of distilled water, at the same temperature, and under the same atmospheric pre ure, will always have the same weight. Advantage has been taken of this property of bodies to connect weights with measures in the metrical systems that have been adopted in this and other countries. English Weights. — It was declared by the Great Charter that the weights should be the same all over England, but no ordinance, perhaps, was ever so ill observed; for the diversity that has prevailed, and which is still far from being remedied, has been so great as not only to produce confusion and inconvenience, but to render the system of weights adopted in one part of the country scarcely intelligible in another. The old English pound, which is said to have been the legal standard of weight from the time of William the Conqueror to that of Henry VII., was derived from the weight of grains of wheat: 32 grains [s. 1334]