METRIC SYSTEM.—The system adopted by the French Convention in 1879, but which is gradually coining into use in this and other countries. The theory of the system is that the meter is a '0,000,000th of a quadrant of the earth through Paris, the liter is a cube of meter, the gram is of the liter filled with water at 4 degs. C.

The meter is a measure of length equal to 39.37 English or American inches.

This system, formed on the meter as the unit of length, has four other leading units, all connected with and dependent upon this. Hence, we have : i. The meter, which is the unit of measures of length.

2. The are, which is the unit of surface, and is the square of the meter.

3. The liter, which is the unit of measures of capacity, and is the cube of a tenth part of the meter.

4. The stere, which is the unit of measures of solidity, having the capacity of a cubic meter.

5. The gram, which is the unit of measures of weight, and is the weight of that quantity of distilled water at its maximum density, fills the cube of a hundreth part of the meter.

Each unit has its decimal multiples and sub-multiples, that is weights and measures ten times larger, or ten times smaller, than the principal units. The prefixes denoting multiples are derived from the Greek, and are : Deka, ten; hecto, hundred; kilo, thousand, and myria, ten thousand. Those denoting sub-multiples are taken from the Latin, and are : Deci, ten; centi,

hundred (as in centigram or centimeter), and milli, thousand.

The metric system has been adopted by many nations, the English excepted. In America its use has been made optional, but is legalized by Congress. All photographic formula received from the continent of Europe express values and quantities with metrical weights and measures. To utilize them directly without translating into the expressions of the English system, the student is advised to procure gram weights and cubic centimeter graduates, and substitute them for those denoting quantities according to the old plan.

As an assistance to those who cannot acquire these aids, we annex tables, which convert grams and cubic centimeters into English grains, drachms, and ounces sufficiently correct for practical purposes.* Metric Fluid cubic centimeter, usually represented by " C.c.," is the unit of the metric measurement for liquids. It contains 17 minims of water. The weight of this quantity of water is i gram. The following table will prove to be sufficiently accurate for photographic purposes :