[105] The draining of the lakes in the hills round Rome is a great mistake, and very injurious to the health of the city. Such lakes are a wise provision of nature for collecting some of the surplus water in the rainy season, and preserving it for use in the hot and dry season, when the evaporation from the lakes helps to cool the air. The water also drawn from these lakes was most useful for the irrigation of the country round Rome, and watering the gardens in Rome itself, thereby promoting vegetation, which is essential for health in a hot and dry climate. It is well known that the leaves of plants and trees (more especially of deciduous trees) absorb _nitrogen_, which is the part injurious to human life, and give out _oxygen_, that portion which is beneficial to, and necessary for, human life. Where there is no vegetation, therefore, the climate cannot be healthy, and without water there can be no vegetation; for water is the necessary food of plants. [106] The line of this subterranean aqueduct can also be traced by the wells descending into the _specus_, in the same manner as the Aqua Appia was traced in 1870, that is, by the bushes growing at the top of each of the wells, and generally enclosed by a wooden railing to prevent animals from falling into them. [107] The rock in which the tunnel is cut is a sort of peperino, hard and rough, covered with a bed of clay. [108] This lake, called Sabatina in the time of Frontinus, was called Anguillara in the Middle Ages, and is now called Bracciano, in both cases from the names of the proprietors. The great family of Anguillara had their origin from this village, of which they were the proprietors, and where they had a castle on the bank of the lake. The present proprietors are the Dukes of Bracciano. The lake produces a great abundance of fish, especially a small fish much resembling the white-bait of London, at least when cooked. [109] There is a small construction over this flood-gate, and at the back of it is this inscription:— ACQVA PAOLA ALLA PRESA DELL’ ACQVA ALSEATINA [110] There are interesting remains of the Necropolis of this ancient city close to the fountain before mentioned, on each side of a deep ravine. On one side, there are nine chambers cut out of the rock on the edge of the cliff, the entrance being in the central chamber, with four others on either side of it. All are full of small square _columbaria_, of very early character. On the opposite side of the ravine, is a similar series of tombs, but in a less perfect state.