in general, is what is wanting, or necessary, to complete some certain quantity or thing.

Arithmetical Complement, is what a number or logarithm wants of unity or 1 with some number of cyphers. It is best found by beginning at the left-hand side, and subtracting every figure from 9, except the last, or right-hand figure, which must be subtracted from 10. So, the arithmetical comp. of the log. 95329714, by subtracting from 9's, &c., is 04670286.

The arithmetical complements are much used in operations by logarithms, to change subtractions into additions, which are more conveniently performed, especially when there are more than one of them in the operation.

Complement, in astronomy, is used for the distance of a star from the zenith; or the arc contained between the zenith and the place of a star which is above the horizon. It is the same as the complement of the altitude, or co-altitude, or the zenith distance.

Complement of the Course, in navigation, is the quantity which the course wants of 90°, or 8 points, viz. a quarter of the compass.

Complement of the Curtain, in fortification, is that part of the anterior side of the curtain which makes the demigorge.

Complement of the Line of Defence, is the remainder of that line, after the angle of the flank is taken away.

Complements of a Parallelogram, or in a Parallelogram, are the two lesser parallelograms, made by drawing two right lines parallel to each side of the given parallelogram, through the same point in the diagonal.

Complement of Life, a term much used, in the doctrine of life annuities, by De Moivre; and, according to him, it denotes the number of years which a given life wants of 86, this being the age which he considered as the utmost probable extent of life. So 56 is the complement of 30, and 30 is the complement of 56.

COMPOSITION or PROPORTION, according to the 15th definition of the 5th book of Euclid's Elements, is when, of four proportionals, the sum of the 1st and 2d is to the 2d, as the sum of the 3d and 4th is to the 4th.

Composition of Ratios, is the adding of ratios together; which is performed by multiplying together their corresponding terms, viz. the antecedents together, and the consequents together, for the antecedent and consequent of the compound ratio; like as the addition of logarithms is the same thing as the multiplication of their corresponding numbers. Or, if the terms of the ratios be placed fraction-wise, then the addition or composition of the ratios is performed by multiplying the fractions together.