WEIGHTS AND MEASURES. Standards of magnitude, of weight, and of value are essen tial for commercial and scientific purposes, and the facility with which national and interna tional intercourse is carried on depends largely upon the uniformity of these standards. Natu rally the units of measure adopted by primitive peoples were varied and imperfect, but as civili zation advanced, trade increased, science devel oped, and more accurate standards became neces sary. Their selection, whether by government action or by common consent, was left almost to chance, so that they have differed from nation to nation, from county to county, from town to town, and even from one trade or guild to another. The last two centuries, however, have witnessed a great advance in favor of uniformity. England, the United States, France, and several other nations have established uniformity in their respective territories, and the Latin nations have practically all adopted the metric system. The same advantage accruing from uniform ity for one o• several countries would evidently obtain if a universal system were adopted by the whole world.
The setting up of a system of measures is fraught with many difficulties. The selection, determination, construction, and copying of the standard furnish peculiar problems. The chief considerations affecting the selection of a stand ard are its relation to a recognized physical constant and the relative ease with whieli these standards may be determined. In the case of the unit of length, two constants have been favored, a fractional part of a terrestrial meridian, and the length of a seconds pendulum in a given locality. The nautical mile of 6080.20 feet or of 60S0 feet is all example of the former and the seconds pendulum of 39.13+ inches an example of the latter. It is quite remarkable that the French mter, based upon the length of a meridian are, should approximate so closely the length of the seconds pendulum. There need be no difficulty in establishing mills of capacity, since these can be based upon the unit of length as exemplified in the system (q.v.).
In the ease of the unit of weight it is neces sary to select a quantity of sonic substance easily obtainable, and easily standardized as io quan tity, purity, and density. Water is a substance which meets these requirements fairly well, and although the British system declares the weight of one cubic inch of distilled water at 02° E. to
be 352.45S grains, and the metric system (q.v.) fixes the kilogram as the weight of one cubic decimeter of water at maximum density, the Anglo-Saxon still uses the troy pound and the avoirdupois poliml (see below).
Although a fortunate selection be made. the determination and construction of a standard are difficult matters. The French engineers spent seven years in determining a kilometer, and even then failed to obtain 0.0001 of the earth's quad rant. The difficulties of establishing a standard pendulum and of computing the lengths of others vibrating in given times are many and great. In the first place. the experiments are made in air, and the buoyancy of the air lessens the actual weight of the pendulum. Then, since the earth has a diurnal motion on its axis, every object placed on it has a centrifugal tendency which modifies what otherwise would be its gravitation. This centrifugal tendency produces the earth's oblateness and causes a variation in the intensity of gravitation from one latitude to another. Thus a stone is actually heavier in Boston than in New York. This change in gravitation can not be measured by a balance because the weights at each end of the balance are changed alike, but it is apparent in the motion of a clock; for a pendulum regulated to beat seconds in Washington is found to go too fast when taken to a higher latitude, and to lose time when car ried nearer to the equator; and again the attrac tion which the earth exerts upon bodies placed near it diminishes with their distances from its centre, being inversely proportional to the squares of the distances, so that a clock car ried from the bottom to the top of a hill loses time perceptibly. In addition to these niceties, there are others connected with the manipu lation, such as the parallelism of the knife-edges, their bluntness, the amplitude of the oscillation, and the stability of the support, so that altogether the exact measurement of the length of the seconds pendulum is a matter of very great difficulty. (See PENDULUM. ) But grant ed that a length be chosen and he expressed as a distance between two lines on a metal bar, the tendency to oxidize in air, to change with tem perature, and to wear with handling, all combat the preservation of the standard and interfere with the process of copying.