Since not all objects have the same dimensions, it becomes necessary to have standards with which different bodies may be compared. The three fundamental units that are used in our daily experiences are the units of time, length, and mass. Without these units it would be impossible to do accurate work or to give and receive working instructions.

The unit of time is the second and is the same in all countries. The day is divided into 24 hours of 60 minutes each, and each minute contains 60 seconds. Twenty-four hours, or one day, is the time taken by the earth to make one complete revolution on its axis. In most trades the hour, minute, and second are used in place of the day as the practical working units of time.

The unit of length by means of which the English-speaking races measure distance is the yard. The standard of length in the British system is the imperial yard. It was defined by an act of Parliament in 1855 as the distance between two cross lines in two gold plugs in a certain bronze bar, kept at 62° Fahrenheit. This bar is preserved at the Board of Trade office in London. Though the unit of length was intended to be the same for England and America, in reality the United States yard exceeds the British by .00087 of an inch. The United States standard yard is the distance between the twenty-seventh and sixty-third inch marks of a scale prepared by the United States Geological Survey.

It is kept at the Bureau of Standards, Washington, D. C.

The foot is one-third of a yard, and the inch one thirty-sixth of a yard.

The units of area and volume are the square and the cube of the unit of length, i.e., the square yard and the cubic yard. The American unit of volume for liquids is the Winchester wine gallon, which contains 231 cu. in.* The British unit is 277.274 cu. in. A quart is one-fourth, a pint (Fig. 1) one-eighth, and a gill one thirty-second of a gallon.

The unit of weight, i.e., mass, is the pound. This weight is based on the force of attraction exerted by the earth upon a block of platinum called a pound weight. This block also is kept in the Board of Trade office in London. The United States standard weight is the avoirdupois pound which is copied from the English measure.