thick extending over the middle third of the depth of the voussoir joints, the rest of the joints being left open.

If the pressure on the lead is uniformly varying, the centre of pressure must be within the middle third of the width of the lead; that is, it cannot deviate from the centre of the voussoir -- Scale of Feet 50 100 200 300 FIG.

voussoir tiles in the hypocaust chamber where we would expect to find stacks of flat tiles.

We found stacks of box flue tiles and voussoir tiles in the hypocaust chamber where we would expect to find stacks of flat tiles.