easy examination of its mode of construction. Flags, about two inches thick, rest upon the pillars; a layer of compost, five inches thick, and formed of lime, sand, gravel, and burned clay or pounded tile, succeeds, and above that, another covering of thin flag-stones.[87] This apartment has been provided with a semicircular recess at its eastern extremity (G), and, at the angle next the street (A), has been supported by a buttress. A similar alcoved recess existed on the western side of one of the principal rooms of the ‘baths’ at HUNNUM, and the same arrangement may yet be observed in the corresponding building at Lanchester. All of these buildings have been strengthened with buttresses, but it is only in these and analogous cases, that the use of the buttress is admitted among the erections of the Barrier; it never occurs in the great Wall or the curtain-walls of the stations. In the circular recess[Sidenote: THE HYPOCAUSTS.] of this apartment is an aperture (G), which probably has served to regulate the current of air circulating in the hypocausts. The furnace which warmed the suite of apartments was situated near the south-east extremity of the building (at F); the pillars near the fire having been much acted upon by the heat, the whole of this part of the floor was reduced, on exposure to the frosts of winter, to the confused heap represented in the drawing. The soot in the flues was found as fresh as if it had been produced by fires lighted the day before.[88] The walls of this apartment were coated with plaster, and coloured dark red; exposure to the weather soon stripped them of this covering. An arched passage curiously turned with Roman tile took the heated air from the furnace through the party-wall (at X) into the chamber to the west of it. The rooms to the westward of the intersecting street (HD), seem to form an independent building, and have less of the aspect of a place of public concourse than the other portions. They may have been the private residence of the commander of the station. They, too, are heated by hypocausts. [Illustration: H. Burdon Richardson, Delt. John Storey, Lith. HYPOCAUST AT CHESTERS, (CILURNUM) Printed by W. Monkhouse, York ] [Sidenote: CILURNUM.]