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COPYRIGHT 2005 Natural History Magazine, Inc.
Give 'em an inch, they'll take a mile.
Kids love to count stuff. Bees, birds, dogs, monkeys, and rats count stuff, too. But if you want to describe objects and phenomena in the physical universe, you must move beyond mere enumeration. You could offer poetic evocations of an object's color, smell, sound, taste, or texture. Or you could quantify its attributes by using the methods and tools of science. But to do science, you must first establish a system of measurement. Fortunately, our technophilic predecessors have already been there and done that.
For thousands of years of human history, the world's farmers, merchants, tax collectors, engineers, and scientists have come up with hundreds of curious units: feet and fathoms, acres and hectares, bales and bushels, troy ounces and metric tons, weeks and fortnights. As knowledge of the universe advanced, more units of measure had to be invented--the kelvin, the pascal, the ampere, the joule, the watt per steradian, the tesla--so that attributes such as temperature, pressure, electric current, energy, radiant intensity, magnetic flux density, and a host of others could be quantified.
Measurements and their attendant units turn pure numbers into physical quantities. Therein lies the foundation of engineering and experimental science, distinguishing these enterprises from pure math, in which a number is just a number. If you knew nothing about units, but you had just gotten acquainted with the "number line," you might think eighty miles an hour is twice as big as a forty-cent bag of peanuts. But those two units have nothing in common, so the comparison is meaningless.
Two physical quantities are comparable only if they can be expressed in the same units: the number 25,000 is certainly bigger than the number 500, yet a giant tortoise chugging along nonstop at 25,000 feet a day moves hopelessly slower than a jet flying 500 miles an hour. Transporting the airplane into the tortoise's universe, 500 miles an hour becomes 63 million feet a day. Tortoise loses.
Once upon a time, life was lived and measured locally, and the actual quantity represented by a given unit--a bushel, a cubit, a foot, a gallon--often differed from place to place. Basic liquid commodities such as wine, beer, and olive oil each had its own unit of measurement. Units of length might be based on...
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