It remains to explain the manner in which the immense body of water required for the supply of a set of baths in the thermæ was heated. This has been done very satisfactorily by Piranesi and Cameron, as may be seen by a reference to the two sectional elevations showing the reservoir and aqueducts belonging to the Thermæ of Caracalla. A are arches of the aqueduct which conveyed the water into the reservoir, B, whence it flowed into the upper range of cells through the aperture at C, and thence again descended into the lower ones by the aperture, D, which were placed immediately over the hypocaust, E, the furnace of which can be seen in the transverse section at F. There were thirty-two of these cells arranged in two rows over the hypocaust, sixteen on each side, and all communicating with one another, and over these a similar number similarly arranged, which communicated with those below by the aperture at D. The parting walls between these cells were likewise perforated with flues which served to disseminate the heat all around the whole body of water. When the water was sufficiently warm it was turned on to the baths through pipes conducted likewise through flues in order to prevent the loss of temperature during passage, and the lower reservoir was supplied as fast as water was drawn off from the reservoir next above, which in turn was supplied with water from the topmost tier and the aqueduct. [Illustration: Frigidarium, Thermæ of Caracalla, Rome. (Restored by Viollet-le-Duc)] Perhaps a better idea of the thermæ can be had by an examination of the plan of the Thermæ of Titus, Rome, restored by Leclerc, also the sectional elevation and front elevation of the same bath, restored by the same artist. The original drawings, which won the _Grand Prix de Rome_, are preserved in the library of the Ecole des Beaux-Arts, Paris. A restoration by Viollet-le-Duc, which appeared with the other restorations in the June, 1906, number of the _Architectural Record_, conveys a very good idea of the interior of a frigidarium. [Illustration] [Illustration: · INTERIOR · VIEW · OF · AQVEDVCT · · LISBON · PORTVGAL] [Illustration: CHAPTER V] SYNOPSIS OF CHAPTER. Fall of the Roman Empire—Succeeding Period known as the Dark Ages—Sanitation during the Dark Ages—Beginning of Material Progress in Sanitation—Pilgrimages to Juggernaut—Water Supply to Paris—London Water Supply—Aqueduct of Zempoala, Mexico.