Momentum,--or quantity of motion. If a body moving at first with a certain velocity is afterwards observed to move with double or triple this velocity, the quantity of motion of the body is conceived to be doubled or tripled, hence the momentum of a body, depends upon its velocity, as the quantity of motion of a body is the product of the velocity by the mass or weight. ~Laws of motion.~ The elementary principles upon which are based all our reasonings respecting the motions of bodies, are called the “Laws of Motion,” and as arranged by Sir Isaac Newton, are three in number. ~1st Law.~ 1st. A particle at rest will continue for ever at rest, and a particle in motion will continue in motion uniformly forward in a straight line, until it be acted upon by some extraneous force. ~2nd Law.~ 2nd. When any force acts upon a body in motion, the change of motion which it produces is proportional to the force impressed, and in the direction of that force. ~3rd Law.~ 3rd. Action and reaction are equal, and in contrary directions. In all cases the quantity of motion gained by one body is always equal to that lost by the other in the same direction. Thus, if a ball in motion, strikes another at rest, the motion communicated to the latter will be taken from the former, and the velocity of the former be proportionately diminished. ~Centre of Gravity.~ Centre of Gravity,--is that point at which the whole weight of the body may be considered to act, and about which consequently, the body, when subjected to the force of gravity only, will balance in all positions. ~Specific Gravity.~ Specific Gravity,--the weight belonging to an equal bulk of every different substance, and is estimated by the quantities of matter when the bulks are the same; or in other words, it is the density that constitutes the specific gravity. It is agreed to make pure rain-water the standard, to which they refer the comparative weights of all other bodies. Lead is about eleven times the weight of the same bulk of water. ~Initial Velocity.~ Initial Velocity is the velocity which a bullet possesses on leaving the muzzle of a gun; and in the speaking of the velocity of bullets fired from the musket now used, you understand 1200 feet per second, for the Initial Velocity. ~Angular Velocity.~ Angular Velocity is the velocity with which the circular arc is described; and depends upon the perpendicular distance of the point from the axis of rotation. ~Terminal Velocity.~ Terminal Velocity: if a cannon ball were to be let fall from a very great height, it would by the law of gravity, descend with accelerated motion towards the earth, but as the resistance of the air increases as the squares of velocities, a point would be reached when the resistance would be equal to the force of gravity, from whence it would fall to the earth in uniform motion. ~Eccentric Body.~