We have already learned the full meaning of the term extension, as applied to matter, and we know that it may be necessary to consider extension in one, two, or three directions-length, surface, volume.
From very early times man must have begun to findit necessary, in dealing with matter of any kind, to get a correct idea of its size. Primitive man no doubt made use of the hand, arm, and foot as natural measures, and perhaps these were then convenient enough, for they were always ready when they were wanted.
Boys nowadays in their games measure distances with the hand, or step them with the feet, and this often leads to quarrelling, because boys" feet and hands are not all the same size, and such measurements cannot be fair and exact.
As it is with boys, so has it always been with men. These rough-and-ready methods of measuring with hand, foot, and arm must have often caused dispute; hence arose the necessity for more fixed and definite standards of measurement. Among the commonest of these very earlymeasures were the span, the cubit, the foot, the inch, and the fathom.
The span was the distance which can be stretched between the thumb and the little finger.
The cubit is stated in ancient manus to be the length of a man"s arm from the elbow to the extremity of the middle finger; it is usually called the natural or common cubit. It is worthy of note that the cubit is the earliest measure of length we meet with, and was in common use throughout the ancient world as the most convenient standard unit of length.
The foot was, of course, the length of a man"s foot; the inch was the breadth of the thumb; and the fathom was the length of the outstretched arms from the tips of the fingers.
These and similar inexact measurements were the standards that prevailed in the ancient world. In course of time, and during the period of confusion between the ancient and modern world, the old standards were lost, but the people, retaining the old names, began to give to each a certain definite length, and to fix that length by law, in order to put an end to disputes.
For example, the average length of the span is 9 of our inches, and of the cubit 18? inches. When therefore we read in the Bible of the cubit and the span, we know that the Jews meant by each of these measurements an actual, recognized, and fixed length. In like manner, when the Greeks and Romans fixed upon the foot as their unit of measurement, they meant by it, not the length of any man"s foot, but a certain fixed length settled by law.
The Romans divided their foot into twelve equal parts, which they called unciae (twelfths), and from this we derive our own word inch, the twelfth part of our foot.
The French afterwards adopted the foot as a unit. It is said that the measure was taken from the foot of the famous Emperor of the West, Charles the Great, and was subsequently adopted as a fixed definite unit of length.
The Saxons adopted the length of a man"s gird or girdle as their unit measure of length, and called it a gird or yard. Men, however, differ very much in their measurement round the waist, and hence, to prevent mistakes and disputes, a certain length was fixed upon for this yard, and the true measure was kept at the royal capital, Winchester.
In the time of Henry I of England another unit was taken, from the length of that king"s arm, and the old name, yard, was given to it. This yard has been the English standard of measure ever since 1135.
The measures in force in the United States were adapted from the English system determined in 1760. Our ancestors were colonists of England, and besides speaking the English language, they had English laws, English customs, and English weights and measures. In this old English system the yard was determined by the length of a pendulum swung under certain conditions in a vacuum at the level of the sea. From this that which is now called the imperial yard was made, and from this yard, theonly authentic representative of the old standard, our measurements are derived. It is a long bronze bar, one inch in thickness, with a plug of gold let in near each end. Through the center of each gold plug a fine cross- line is cut; the distance between the two fine cross-lines is the true yard. The bar is always to be measured at one temperature, 62°F.
From the yard, as the standard unit, are formed the various multiples and sub-multiples, which give us our complete measure of length.