The ability to measure accurately, and thus obtain a definite and positive knowledge, instead of a general and indefinite knowledge of form, relation, distance, and the other phenomena of the existing condition of things in which we are placed, constitutes the difference between scientific knowledge and ordinary knowledge. By some philologists our term man is traced to a derivation from the Aryan root-word ma, to measure. Whether this derivation is true or not, certain it is that the most accurate and comprehensive definition of man, as classified at the head of the organic evolution of intelligence upon this planet, is that of a measurer, and, as symbols of his true domination of the world, a rule and a pair of scales would be much fitter and more expressive of his glory than a crown and a sceptre.
The use of the rule is so absolutely necessary in almost every mechanical or artistic pursuit, that the consumption is, of course, very great, and the manufacture is consequently a very important one. Rules are generally made of boxwood or of ivory, and are mounted and tipped with brass or silver. Boxwood is most extensively used, both on account of its being more plentiful than ivory, and also because it is less liable to expand and contract by variations of the temperature. This last consideration is the most important, since the accuracy of the rule depends upon the constancy with which it marks the fixed standard for lineal measurement.
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