The work of writing the solution may be lessened by omitting the

8. The compass. So far segments have been measured with a ruler alone. Line segments may be measured with a compass (Fig. 25) as follows:

Open the compass and place the sharp points at A and B, the end points of the segment AB. Then place

the points on the marks of a ruler (Fig. 26) and count to the nearest sixteenth of an inch the number of inches between them. This is the length of AB. Why?

By this method of measuring with the compass one is able to measure more accurately than with the ruler alone. For, when we measure with the ruler the eye

has to pass from the end points of the segment to the marks on the ruler, which makes it difficult to get the best reading. The error is reduced by carrying with the compass the distance between the end points from segment to ruler. Hence, when exact work is required the compass should be used. It is important to keep the pencil point of the compass sharpened.

C

FIG. 27

EXERCISES

1. Using the compass and ruler as shown above measure AB, BC, and CA (Fig. 27) to the nearest sixteenth of an inch.

2. The drawings (Figs. 28 and 29) represent a match safe and the designs (working drawings) for making the safe. A length of an

inch in the design represents 2 inches in the corresponding length of the safe. Find to the nearest sixteenth of an inch the following dimensions of the safe: width, height, depth, and thickness of boards.

9. How standard units of length have been made. Most of the civilized nations have derived a unit of length from the length of the human foot. The result has been that standard units of length are not the same in different countries.

Early units of measurements were indefinite. Thus the foot-breadth and cubit (distance from the elbow to the extremity of the middle finger) mentioned in the Bible vary for different persons, and exact measurement with these units is impossible. Hence it became necessary for people to adopt a definite unit.

In a book on surveying we find the following account of how people in the sixteenth century tried to obtain a standard unit. "Stand at the door of a church on Sunday, and bid sixteen men to stop, tall ones and small ones, as they happen to pass out when service is finished; then make them put their left feet one behind the other and the length thus obtained shall be a right and lawful rod to measure and survey land with, and the sixteenth part of it shall be a right and lawful foot."1

Henry I, King of England (1100-1135 A.D.), is said to have used as a unit the distance from the point of his nose to the end of his thumb, approximately a yard's length. Other units were made in 1490 by Henry VIII and in 1588 by Queen Elizabeth. The present English standard was adopted in 1855.

In 1856 the English Government sent to this country two copies of the new English standard, one made of bronze, the other of iron, which were used as standards

1 Lessons in Community and National Life. Series B, Chapter VI. By Charles H. Judd and Leon C. Marshall.

until 1875. The table below gives the units of linear measure in that system.

During the French Revolution the National Assembly (1790) appointed a committee of the Academy of Sciences to study the matter of finding a suitable system of weights and measures. This commission selected as the standard unit of length one tenmillionth part of the distance from the north pole to the equator, measured along the meridian through Paris. This standard unit is called meter. The commission determined the length of a meter to be about 39.37 inches, the work requiring seven years for its completion. This distance was marked off by expert instrument makers on a bar of platinum. On June 22, 1799, the standard unit was presented to the Council of Five Hundred and deposited in the archives at Paris. It is the only unit of length which is the result of scientific investigation. The meter (m.) is divided into 10 equal parts called decimeters (dm.). Each decimeter is divided into 10 equal parts called centimeters (cm.). Each centimeter is divided into 10 equal parts called millimeters (mm.). A thousand meters make a kilometer (km.). The following table expresses these lengths in terms of inches, feet, yards, and miles.

We shall speak of this system of measures as the metric system. It is now generally used throughout the world in scientific investigations. In view of the trade between nations it is desirable that they employ a uniform system of measures in commerce. In 1866 a law was passed making the metric system legal in the United States. The system is easier to understand and to use than our common system. Fig. 30 represents

1 2

3 4 5 6 7

FIG. 30

a part of a ruler graduated according to the metric system.

The following editorial, taken from one of our leading newspapers, describes the merits and importance of the metric system, in urging its adoption:

ADOPT THE METRIC SYSTEM

One of the bills which ought to be passed by congress is the Britten-Ladd bill, which provides for a gradual adoption of the metric system in the United States. The measure has been well considered and is conservative in its provisions. It provides for a