The Centimetre is equal to 0.3937 inch in length, and nominally represents one thousand-millionth part, or 1/1,000,000,000 of a quadrant of the earth. The Gramme is equal to 15.432 grains, and represents the mass of a cubic centimetre of water at 4 degrees C. Mass is the quantity of matter in a body. The Second is the time of one swing of a pendulum making 86,164.09 swings in a sidereal day, or 1/86,400 part of a mean solar day. II. DERIVED MECHANICAL UNITS.- Area.-The unit of area is the square centimetre. Volume.--The unit of volume is the CUBIC CENTIMETRE. VELOCITY is rate of change of position. It involves the idea of direction as well as that of magnitude. VELOCITY is UNIFORM when equal spaces are traversed in equal intervals of time The unit of velocity is the velocity of a body which moves through unit distance in unit time, or the VELOCITY OF ONE CENTIMETRE PER SECOND. MOMENTUM is the quantity of motion in a body, and is measured by mass x velocity. ACCELERATION is the rate of change of velocity, whether that change take place in the direction of motion or not. The unit of acceleration is the acceleration of a body which undergoes unit change of velocity in unit time, or an acceleration of one centimetre-per-second per second The acceleration due to gravity is considerably greater than this, for the velocity imparted by gravity to falling bodies in one second is about 981 centimetres per second (or about 32.2 feet per second). The value differs slightly in different latitudes. At Greenwich the value of the acceleration due to gravity is g=981.17; at the Equator g=978.1; at the North Pole g=983.1. FORCE is that which tends to alter a body's natural state of rest or of uniform motion in a straight line. FORCE is measured by the acceleration which it imparts to mass--i. e., mass x acceleration. THE UNIT OF FORCE, or DYNE, is that force which, acting for one second on a mass of one gramme, gives to it a velocity of one centimetre per second. The force with which the earth attracts any mass is usually called the "weight" of that mass, and its value obviously differs at different points of the earth's surface The force with which a body gravitates--i e, its weight (in dynes), is found by multiplying its mass (in grammes) by the value of g at the particular place where the force is exerted. Work is the product of a force and a distance through which it acts. The unit of work is the work done in overcoming unit force through unit distance--i e, in pushing a body through a distance of one centimetre against a forch of one dyne. It is called the Erg. Since the "weight" of one gramme is 1 X 981 or 981 dynes, the work of raising one gramme through the height of one centimetre against the force of gravity is 981 ergs or g ergs. One kilogramme-metre = 100,000 (g) ergs = 9 8 1 X 10^7 ergs. One foot- pound = 13,825 (g) ergs, = 1 356 X 10^7 ergs.