Abscissa

in military mathematics, signifies any part of the diameter or axis of a curve, contained between its vertex or some other fixed point, and the intersection of the ordinate. In the parabola, the absci a is a third proportional to the parameter and the ordinate. In the ellipsis, the square of the ordinate is equal to the rectangle under the parameter and absci a , le ened by another rectangle under the said absci a , and a fourth proportional to the axis, the parameter, and the absci a . In the hyperbola, the squares of the ordinates are as the rectangles of the absci a by another line, compounded of the absci a and the transverse axis. But it must be remembered, that the two proportions relating to the ellipsis and hyperbola, the origin of the absci as , or point from whence they began to be reckoned, is supposed to be the vertex of the curve, or, which amounts to the same thing, the point where the axis meets it; for if the origin of the absci a be taken from the centre, as is often done, the above proportions will not be true.