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O tribe of savages has ever been found that did not present some evidence of the existence of individual property among them. By force of character and personal prowess the chief acquires possessions of increasing variety. Where compulsion can not be directly applied resort is had to exchange, and this at once develops the need for measurement of values. Local convenience suggests conventional standards for the measurement of quantity, and custom tends to fix such standards. When a number of tribes have become aggregated into an embryonic nation, the different standards are soon found to need revision. From a group of temporary standards some fall into disuse and the most convenient are retained. The readiest standard of length is some part of the human body, such as the forearm or hand. The cubit is thus one of the most ancient of units. The foot, the pace, the palm, the digit, the inch as the length of the last bone of the thumb, the yard as arm length from mouth to finger tip, all of these are units of unknown antiquity, and accurate enough for the common needs of many who are moderately civilized to-day.

The unit of length is the primary unit to which finally all others are referred. To derive from it units of surface and volume would appear most natural, and it seems but a short step farther to derive a unit of mass from the unit volume of some selected kind of matter, such as water or earth. But it is safe to say that such a process of derivation was unknown until within the last few centuries or even less. For the comparison of masses scales were early developed, and with the advance of civilization linear units derived from human bodies of variable size gave place to metallic standards prepared and kept by some central authority. From the buried city of Pompeii have been taken steelyards carrying inscriptions which showed that they had been 'proved' by comparison with the standards kept in the Temple of Jupiter at Rome.

In England the standard of length during the last eight or nine centuries has been the yard, traditionally derived from the length of the arm of King Henry I. about the year 1101. A rod or bar of this length was kept in London, and copies of it, of various grades of crudeness, received the royal stamp which made them legal measures. One third of this length was called a foot, although about one fifth