Here the Government of France took the lead; in 1791 it appointed as a committee Lagrange, Laplace, Borda, Monge, and Condorcet, five illustrious members of the French Academy, to choose a natural constant from which a unit of measurement might be derived, that constant to serve for comparison or reference at need. They chose the world itself to yield the unit sought, and set on foot an expedition to ascertain the length of a quadrant, or quarter-circle of the earth, from the equator to the north pole, taking an arc of the meridian from Dunkirk to Barcelona, nearly nine and one-half degrees, as part of the required curve. When the quadrant had been measured, with absolute precision, as it was believed, its ten-millionth part, the _metre_, was adopted as the new standard of length. As the science and art of measurement have since advanced, it has been found that the measured quadrant is about 1472.5 metres longer than as reported in 1799 by the commissioners. Furthermore, the form of the earth is now known to be by no means the same when one quadrant is compared with another; and even a specific quadrant may vary from age to age both in contour and length as the planet shrinks in cooling, becomes abraded by wind and rain, rises or falls with earthquakes, or bends under mountains of ice and snow in its polar zones. All this has led to the judicious conclusion that there is no advantage in adopting a quadrant instead of a conventional unit, such as a particular rod of metal, preserved as a standard for comparison in the custody of authorities national or international. What gives the metric system pre-eminence is the simplicity and uniformity of its decimal scale, forming part and parcel as it does of the decimal system of notation, and lending itself to a decimal coinage as in France, Germany, Italy, and Spain. The metre is organically related to all measures of length, surface, capacity, solidity, and weight. A cubic centimetre of water, taken as it melts in a vacuum, at 4° C., the temperature of maximum density, is the _gram_ from which other weights are derived; this _gram_ of water becomes a measure of capacity, the millilitre, duly linked with other similar measures. Surfaces are measured in square metres, solids in cubic metres. Simple prefixes are: deci-, one-tenth; centi-, one-hundredth; milli-, one-thousandth; deka-, multiplies a unit by ten; hecto-, by one hundred; kilo-, by one thousand; and myria-, by ten thousand. As long ago as 1660 Mouton, a Jesuit teacher of Lyons, proposed a metric system which should be unalterable because derived from the globe itself. Watt, the great improver of the steam engine, in a letter of November 14th, 1783, suggested a metric system in all respects such as the French commissioners eight years later decided to adopt.