Propositions
Proposition 1.
To find the center of a given circle.
Corollary. If in a circle a straight line cuts a straight line into two equal parts and at right angles, then the center of the circle lies on the cutting straight line.
Proposition 2.
If two points are taken at random on the circumference of a circle, then the straight line joining the points falls within the circle.
Proposition 3.
If a straight line passing through the center of a circle bisects a straight line not passing through the center, then it also cuts it at right angles; and if it cuts it at right angles, then it also bisects it.
Proposition 4.
If in a circle two straight lines which do not pass through the center cut one another, then they do not bisect one another.
Proposition 5.
If two circles cut one another, then they do not have the same center.
Proposition 6.
If two circles touch one another, then they do not have the same center.
Proposition 7.
If on the diameter of a circle a point is taken which is not the center of the circle, and from the point straight lines fall upon the circle, then that is greatest on which passes through the center, the remainder of the same diameter is least, and of the rest the nearer to the straight line through the center is always greater than the more remote; and only two equal straight lines fall from the point on the circle, one on each side of the least straight line.
Proposition 8.
If a point is taken outside a circle and from the point straight lines are drawn through to the circle, one of which is through the center and the others are drawn at random, then, of the straight lines which fall on the concave circumference, that through the center is greatest, while of the rest the nearer to that through the center is always greater than the more remote, but, of the straight lines falling on the convex circumference, that between the point and the diameter is least, while of the rest the nearer to the least is always less than the more remote; and only two equal straight lines fall on the circle from the point, one on each side of the least.
Proposition 9.
If a point is taken within a circle, and more than two equal straight lines fall from the point on the circle, then the point taken is the center of the circle.
Proposition 10.
A circle does not cut a circle at more than two points.
Proposition 11.
If two circles touch one another internally, and their centers are taken, then the straight line joining their centers, being produced, falls on the point of contact of the circles.
Proposition 12.
If two circles touch one another externally, then the straight line joining their centers passes through the point of contact.
Proposition 13.
A circle does not touch another circle at more than one point whether it touches it internally or externally..
Proposition 14.
Equal straight lines in a circle are equally distant from the center, and those which are equally distant from the center equal one another.
Proposition 15.
Of straight lines in a circle the diameter is greatest, and of the rest the nearer to the center is always greater than the more remote.
Proposition 16.
The straight line drawn at right angles to the diameter of a circle from its end will fall outside the circle, and into the space between the straight line and the circumference another straight line cannot be interposed, further the angle of the semicircle is greater, and the remaining angle less, than any acute rectilinear angle.