QUANTITY

, denotes any thing capable of estimation, or mensuration; or which being compared with another thing of the same kind, may be said to be either greater or less, equal or unequal to it.

Mathematics is the doctrine or science of Quantity.

Physical or Natural Quantity, is of two kinds: 1st, that which nature exhibits in matter, and its extension; and 2dly, in the powers and properties of natural bodies; as gravity, motion, light, heat, cold, density, &c.

Quantity is popularly distinguished into continued and discrete.

Continued Quantity, is when the parts are connected together, and is commonly called magnitude; which is the object of geometry.

Discrete Quantity, is when the parts, of which it consists, exist distinctly, and unconnected; which makes what is called multitude or number, the object of arithmetic.

The notion of continued Quantity, and its difference from discrete, appears to some without foundation. Mr. Machin considers all mathematical Quantity, or that for which any symbol is put, as nothing else but number, with regard to some measure, which is considered as 1; for that we know nothing precisely how much any thing is, but by means of number. The notion of continued Quantity, without regard to some measure, is indistinct and confused; and though some species of such Quantity, considered physically, may be described by motion, as lines by the motion of | points, and surfaces by the motion of lines; yet the magnitudes, or mathematical Quantities, are not made by the motion, but by numbering according to a measure. Philos. Trans. numb. 447, pa. 228.

Quantity of Action. See Action.

Quantity of Curvature at any point of a curve is determined by the circle of curvature at that point, and is reciprocally proportional to the radius of curvature.

Quantity of Matter in any body, is its measure arising from the joint consideration of its magnitude and density, being expressed by, or proportional to the product of the two. So, if M and m denote the magnitude of two bodies, and D and d their densities; then DM and dm will be as their Quantities of matter.

The Quantity of matter of a body is best discovered by its absolute weight, to which it is always proportional, and by which it is measured.

Quantity of Motion, or the Momentum, of any body, is its measure arising from the joint consideration of its Quantity, and the velocity with which it moves. So, if q denote the Quantity of matter, and v the velocity of any body; then qv will be its quantity of motion.

, in Algebra, are the expressions of indefinite numbers, that are usually represented by letters. Quantities are properly the subject of Algebra; which is wholly conversant in the computation of such Quantities.

Algebraic Quantities are either given and known, or else they are unknown and sought. The given or known Quantities are represented by the first letters of the alphabet, as a, b, c, d, e, &c, and the unknown or required Quantities, by the last letters, as z, y, x, w, &c.

Again, Algebraic Quantities are either positive or negative.

A positive or affirmative Quantity, is one that is to be added, and has the sign + or plus prefixed, or understood; as ab or + ab. And a negative or privative Quantity, is one that is to be subtracted, and has the sign - or minus prefixed; as - ab.