The tetrastyle atrium differed from the Tuscan in only one respect: there were four columns supporting the roof, one at each corner of the impluvium. In most cases these supports, which interfered with the view of the interior, can hardly have been intended primarily for ornament; they simplified the construction, making the ceiling and roof firm without the use of the heavy and expensive girders. The Corinthian atrium had a larger compluvium than the other kinds, the roof being supported by a number of columns. There are three examples at Pompeii, the houses of Epidius Rufus with sixteen columns (p. 310), of Castor and Pollux with twelve, and of the Fullonica with six. The roof of the displuviate atrium sloped from the middle toward the sides, the water being carried off by lead pipes. The aperture for the admission of light and air was relatively much higher above the floor than in the kinds previously described. No example of this type has been found at Pompeii. [Illustration: Fig. 118.--A Tuscan atrium: section. _b._ Girder. _e._ Corner rafter. _h._ Impluvium. 1. Flat tiles. 2. Semicylindrical tiles.] The tortoise atrium, _atrium testudinatum_, was small and without a compluvium. The roof had a pyramidal shape. There were possibly a few examples at Pompeii, as we may infer from the occasional absence of an impluvium; in the only instance, however, in which it is possible to determine the form of the roof (V. v. 1-2), this must have been very different from that referred to by the Roman writer (p. 343). Vitruvius says further that the atrium should have an oblong shape, the width being three fifths or two thirds of the length, or measured on the side of a square, the hypothenuse of which is taken for the length. The design was obviously to bring the sides nearer together, thus lessening the strain on the two girders which in the commonest form were used to sustain the roof. The height, to the frame of the compluvium, should be three fourths of the width. In the case of the tetrastyle and Corinthian atriums at Pompeii the height is indicated by that of the columns, but there are rarely adequate data for determining the height of the others with exactness. In regard to length and breadth the proportions harmonize fairly well with those recommended by Vitruvius; but the height, in the cases in which it can be ascertained, is often greater than that contemplated by the rules of the architect. Looking at the Pompeian atriums in their present condition (Plate VII, Figs. 121, 153) one might easily receive the impression that they were primarily courts rather than rooms. In this respect the restorations of Roman houses in the older books are often at fault, the atrium being generally represented as too low in comparison with the rooms around it. [Illustration: Fig. 119.--Corner of a compluvium with waterspouts and antefixes, reconstructed.]