The Metric System
Every country in the world uses the metric system although many products are still manufactured in common sizes for public use. The metric system was devised by French scientists in the late 18th century to replace the chaotic collection of units then in use. The goal of this effort was to produce a system that did not rely on a miscellany of separate standards, and to use the decimal system rather than fractions.
To obtain a standard of length a quadrant of the earth (one-fourth of a circumference) was surveyed from Dunquerque in France to Barcelona in Spain along the meridian that passes through Paris. The distance from the North Pole to the equator was divided into ten million parts to constitute the meter (spelled metre in some countries). The definition of the meter has become more and more precise through the years since, even though its length has not changed. Currently the meter is the distance light travels in a vacuum in 1/299 792 458 second.
The nautical mile used in modern navigation, in relation to which boat speeds and wind velocities are measured (one knot is one nautical-mile-per-hour), is approximately one minute of latitude. A degree of latitude therefore is about 60 nautical miles. The quadrant of the earth measured by the French, being 90 degrees, measures 90x60 or 5400 nautical miles. Therefore: 5400 nautical miles equal about 10-million meters, or 10 000 kilometers.
Aviation maps (WAC), scaled to one-millionth actual size, can be measured with an ordinary "ruler." One millimeter on the map equals one kilometer on the ground. Curiously, one sixteenth of an inch on the same map represents almost exactly one statute mile on the ground (within 1.38%).
On April 5, 1893 the inch was redefined as precisely 1/39.37 meter, and in a very real sense we have been using the metric system ever since. In 1959 the length of the inch was shortened slightly to its present definition of 2.540 000 000 centimeters.
The centimeter is about the width of your little finger. The "hand" (used for measuring horses) is about 10 centimeters, or a decimeter.
The speed of light, a universal constant, is almost exactly 300 megameters per second.
The circumference of the earth is 40 megameters (by the original definition of the meter in the eighteenth century).