There's only pressure on the stones of a circular arc. But on the supporting points of the arc there's a resulting oblique force that can be devided in a vertical and a horizontal force.[br]Next applet illustrates the width of the wall that's needed to support this horizontal force.

Adjacent arcs support the horizontal forces of each other. A magnificent example is the is the Pont du Gard (1st century) 49m high, near to the city of Nîmes. On the outsides of the aquaduct the horizontal forces are supported by the river banks.

Putting extra material on the outsides of the arc can reduce the horizontal forces by verticalising them. Not supported arcs tended to break regularly at a height corresponding with an angle of 30° to 35°.