The only London vault which I can find mentioned is one found exactly two hundred years since at St. Martin-in-the-Fields. A Minute of the Society of Antiquaries reads: “May 2, 1722: Mr. Stukely related that the Roman building in St. Martin’s Church was an arch built of Roman brick and at the bottom laid with a most strong cement of an unusual composition, of which he has got a lump. There was a square duct in each wall its whole length, of 9 in. breadth; there were several of these side by side: this building is below the springs on the gravel.” This building that was an arch, with its many flues, and cement floor—doubtless _opus signinum_—was obviously a Roman bath chamber, but probably it was quite small. [Illustration: FIG. 5. ] Evidence of the existence of fairly large vaults has been found at the Baths of Silchester, Wroxeter and Bath. These were all constructed in a most interesting and suggestive way of voussoirs made as hollow boxes in the tile material. Similar box voussoirs have been discovered at Chedworth and elsewhere. [Illustration: FIG. 6. ] [Illustration: FIG. 7. ] [Illustration: FIG. 8. ] I have found two such box voussoirs in the Rochester Museum, each about 9 in. by 6 in. on the face and 5 in. on the soffit (Fig. 4). The surfaces are roughly scored across with parallel lines forming an X. These two tiles together show an obvious curvature; they came from a villa at Darenth. In the Guildhall Museum I have also found a box voussoir which is almost identical with those at Rochester. It is thus described: “74, Flue (?) tile, red brick, the front decorated with incised cross lines; in the centre both front and back is a circular perforation: 9½ in. long, 6¾ in. high, 6½ in. wide.” The longest dimension is not in the direction of the tube, and the height is greater at one end than the other, so that the wedge form is quite apparent. The small holes in both the larger sides were doubtless to give better hold to the mortar in which they were set (Fig. 5). Roach Smith recorded what must have been broken parts of similar voussoirs as found in Thames Street in 1848 (_Journ. Brit. Archæol. Assoc._, vol. iv.), but here they seem to have been used as waste material in building the little piers of hypocausts. Roman builders also constructed vaults of pipes and pots set in mortar concrete as were our box voussoirs, but I know of no British examples. Vaults of wide span seem to have covered large chambers in the Basilica at Verulam (see _Victoria County History_). The method of using the box voussoirs has been well explained from the Silchester examples by the late Mr. Fox in _Archæologia_ (cf. Fig. 6). A fragment at Westminster Abbey is either part of a voussoir or of a short flue tile (Fig.7).