If weights be suspended from various points of a hang ing chain, the intervening por tions will form arcs of equal ~ **catenaries**, since the horizontal tension (wa) is the same for all.

Again, if a chain pass over a perfectly smooth peg, the **catenaries** in which it hangs on the two sides, though usually of different parameters, wifi have the same directrix, since by (10) y is the same for both at the peg.

Hence for every tension greater than the minimum tension there are two **catenaries** passing through A and B.

Since the tension is measured by the height above the directrix these two **catenaries** have the same directrix.

Now let us consider the surfaces of revolution formed by this system of **catenaries** revolving about the directrix of the two **catenaries** of equal tension.

The **catenaries** which lie between the two whose direction coincides with the axis of revolution generate surfaces whose radius of curvature convex towards the axis in the meridian plane is less than the radius of concave curvature.

The **catenaries** which lie beyond the two generate surfaces whose radius of curvature convex towards the axis in the meridian plane is greater than the radius of concave curvature.

14) be two **catenaries** having the same directrix and intersecting in A and B.

Draw Pp and Qq touching both **catenaries**, Pp and Qq will intersect at T, a point in the directrix; for since any **catenary** with its directrix is a similar figure to any other **catenary** with its directrix, if the directrix of the one coincides with that of the other the centre of similitude must lie on the common directrix.