QUANTITY, any thing capable of es timation, or mensuration ; or which, being compared with another thing of the same kind, may be said to be greater or less than it, equal or unequal to it. Mathema tics is the science or doctrine of quantity, which being made up of parts, is capable of being made greater or less. It is in creased by addition, and diminished by subtraction ; which are therefore the two primary operations that relate to quanti ty. Hence it is that any quantity may be supposed to enter into algebraic compu tations two different ways, which have contrary effects, viz. either as an incre ment or as a decrement.
A quantity that is to be added, is called a positive quantity ; and a quantity to be subtracted, is said to be negative. Quan tities are said to be like or similar, that are of the same denomination, they are represented by the same letter or letters, equally repeated : but quantities of differ ent denominations, or represented by a different letter or letters, are said to be unlike or dissimilar. A quantity consist ing of more than one term is called a compound quantity ; whereas that con.
sisting of one term only is denominated a simple quantity.
The quantity of matter in any body, is the product of its density into its bulk ; or a quantity arising from the joint consi deration of its magnitude and density ; as if a body be twice as dense, and take up twice as much space as another, it will be four times as great. This quantity of mat ter is best discoverable by the absolute weight of bodies.
The quantity of motion in any body is the factum of the velocity into the mass, or it is a measure arising from the joint consideration of the quantity of matter, and the velocity of the motion of the bo dy ; the motion of any whole being the sum or aggregate of the motion in all its several parts. Hence, in a body twice as great as another, moved with an equal ve locity, the quantity of motion is double; if the velocity be double also, the quan tity of motion will be quadruple. Ile riCe the quantity of motion is the same with what we call the momentum or impetus of a moving body.