The hydraulic properties of the concrete used in the floors of Roman hypocausts, has, I believe, escaped the notice of previous writers, and is the only other point which need longer detain us in this interesting building. My attention was drawn to this subject by my brother, Mr. George Barclay Bruce, Member of the Institution of Civil Engineers, in the following communication:— In many places on the line of the Wall, the mortar has had mixed with it broken tiles or burned clay, to assist it in resisting the moisture of the atmosphere. The concrete at Chesters placed between the slabs of the bath-room, has a very large proportion of this burned clay, and would thus be better suited to resist the action of heat below and water above than purer lime. A portion of this concrete was taken, by way of experiment, and burned in a crucible, as though it had been a piece of limestone; it was then ground fine, and mixed with a proper quantity of water; after being allowed to dry for three or four hours, it was immersed in water, where it set in the same manner as common mortar does in the open air, clearly proving its hydraulic properties. The same experiment was tried with the ordinary mortar of the Wall, but without the same result, there not being a sufficiently large quantity of burned clay to enable it to stand so severe a test. In the case of the concrete, it did not set so readily as what is called Roman cement, but sufficiently so to prove that it is a strong hydraulic mortar, made by the mixture of burned clay with common lime. [Sidenote: THE CEMETERY.]