To a common observer the difficulties of finding an unvarying standard are not readily apparent. But philosophy shows that all things in Nature are undergoing change; so that there would seem to be no invariable magnitude, the same in all countries and in all times, as Cicero pictured the great principles of Natural Law,[56] by which a lost standard on an inaccessible island might be reproduced with mathematical certainty. There is but one magnitude in Nature which, so far as we know, approximates to these requisites. I refer to the length of the pendulum vibrating seconds, which in our latitude is about 39.1 inches. This length, however, varies in travelling from the equator to the pole, and it also varies slightly under different meridians and the same latitude; but the law of variation has been determined with considerable accuracy. One element in this variation is the difference of temperature. In his report on weights and measures, Mr. Jefferson proposed that we should find our standard in the pendulum. At the same time, the French Government, just struggling to throw off ancestral institutions, conceived the idea of a new system, which, founded in science, should be common to the civilized world. The French began not only by discarding old systems, but also by discarding a measure derived from the pendulum. They conceived the idea of measuring an arc of the earth’s meridian, and finding a new unit in a subdivision of this immense span. The work was undertaken. An arc of the meridian, embracing upward of nine degrees of latitude, and extending from Dunkirk, in France, to the Mediterranean, near Barcelona, in Spain, was measured with scientific care. Illustrious names in French science, Méchain and Delambre, were engaged in the work, which proceeded, notwithstanding domestic convulsion and foreign war. The Reign of Terror at home and invasion from abroad did not arrest it. Seven years elapsed before the measurements were completed, when other nations were invited to coöperate in the establishment of the new system. The unit of measure was one ten-millionth part of the distance between the equator and the north pole thus measured. It received the name of _metre_, from the Greek, signifying _measure_. A bar of platinum, representing this length, was prepared with all possible accuracy. This bar was deposited in the archives of France as the perpetual standard. Other bars have been copied from it and distributed throughout France and in foreign countries.