circus.] A piane figure contained by one line called the circumference, and such that all straight lines drawn front a point within the figure, called the centre, to the circumference are equal. Any straight line drawn through the centre of a circle, and terminated both ways by the circumference, is called a diameter. A line from the centre to the circumference is called a radius. Two diameters at right angles to one another divide a circle into four equal parts called quadrants. Each of these is divided into 900, so that the whole circumference of a circle contains 3600.