RATIONAL

, in Arithmetic &c, the quality of numbers, fractions, quantities, &c, when they can be expressed by common numbers; in contradistinction to irrational or surd ones, which cannot be expressed in common numbers. Suppose any quantity to be 1; there are infinite other quantities, some of which are commensurable to it, either simply, or in power: these Euclid calls Rational quantities. The rest, that are incommensurable to 1, he calls irrational quantities, or surds.

Rational Horizon, or True Horizon, is that whose plane is conceived to pass through the centre of the earth; and which therefore divides the globe into two equal portions or hemispheres. It is called the Rational horiz on, because only conceived by the understanding; in opposition to the sensible or apparent horizon, or that which is visible to the eye.