In the time of the Early Empire it became the fashion to bathe with very warm water. 'People want to be parboiled,' Seneca exclaims. The construction of the alveus, however, was not well adapted to conserve the heat, and an ingenious contrivance was devised to remedy the difficulty, which may best be explained with the help of our illustration, showing the arrangement of the bath basin in room 4 (Fig. 88). A large hot air flue, D, led directly from the furnace to the hollow space, C, under the alveus, A. Above this flue was a long bronze heater, B, in the form of a half cylinder, with one end opening into the end of the alveus. As the bottom of the heater was six inches lower than that of the alveus, the cooler water from the basin would flow down into it and be heated again, a circulation being thus maintained. [Illustration: Fig. 88.--The bath basin in the women's caldarium--longitudinal and transverse sections, showing the arrangement for heating the water. A. Bath basin, alveus. B. Bronze heater. C. Hot air chamber under the floor. D. Hot air flue.] A similar arrangement (called _testudo alvei_ by Vitruvius) probably existed for the alveus in the caldarium on the other side; but that part of the men's baths has been destroyed. Only one other heater of this kind has been found,--and that much smaller,--in a villa near Boscoreale, recently excavated; but the semicircular opening made for the heater above the hot air flue may be seen in the Central Baths, in a private establishment at Pompeii, and generally in the remains of Roman baths. In the furnace room (_praefurnium_, IX) between the two caldariums, stood three large cylindrical tanks. They have disappeared, but their outlines can still be seen in the masonry of the foundations, and are shown in our plan. The one furthest east was for hot water. It was directly over the fire, and connected with the bath basins of the two caldariums. The next, for lukewarm water, stood over a hollow space opening into the furnace. A lead pipe leading from it to the labrum of the women's caldarium is still to be seen; the water bubbled up in the middle of the labrum. The third and largest reservoir, for cold water, was placed on a foundation of solid masonry. The more important alterations made in the baths during the two centuries that they were in use had to do with the arrangements for heating, and may briefly be considered here before we proceed to another part of the building. It will be best not to weary the reader with details, but to present a brief summary of conclusions, which will perhaps be found of interest, not only as casting light on the gradual development of these baths, but also as illustrating that adjustment of public buildings to the needs and tastes of successive generations, which was as characteristic of ancient as it is of modern life.