These and other passages in the work of Frontinus shew that great works were carrying on under his direction, not then completed, their object being to improve the quality of the water, and to supply an additional quantity in case any of the existing aqueducts should fail, as they did occasionally, and to increase the power of distributing it, with a view to improving the climate. This is a proof that Malaria existed at that time[174], and that the best remedy for it was an abundant supply of water in the hot weather, for which the subterranean aqueducts were most useful. The improvement of the Aqua Alsietina, and the addition of the Sabatina, were not sufficient for both these purposes; and, as they supplied the district of the Trastevere only, this could not be all that was intended by Frontinus in describing the great works begun by him under Nerva, carried on by Trajan, and completed by Hadrian. The only work which will correspond to this account is the great aqueduct called by modern writers the Aqua Alexandrina, and erroneously attributed to Alexander Severus only. The sources of this aqueduct are about three miles from Gabii in a watery meadow, nearly under La Colonna, the ancient Labicum. The reservoir of the Aqua Felice is in the same meadow, very near that of Hadrian. Several of the springs that supplied that of Hadrian were intercepted by the engineers of the Aqua Felice, who mistook these springs for those of the Aqua Marcia, shewing their ignorance of the line of the aqueducts. One of the streams, which is rapid and has a considerable body of water, is not used, because the water is of bad quality, and is so full of chalk that it is a petrifying stream. The first central _castellum_ and _piscina_ of Hadrian remains nearly intact, lined with the usual brickwork of that period, but much disguised in outward appearance. It is divided in the inside by a rough stone wall, as was frequently the case; the upper chamber to it has an external staircase added, and windows pierced in it, giving it now the appearance of a mere farm-house. Here the _specus_ is perfect, and from hence it goes at first on a substructure, then on an arcade across the fields, and along the line of arcade to the Cento Celle. The part near the reservoir is destroyed, as is the opposite end near that place, but great part of the arcade in the intervening portion remains nearly perfect, and is one of the finest arcades of the aqueducts, extending for miles across the country between the Via Gabina and the Via Labicana. In some places it is double, one arcade over another, to cross a low valley or a stream. At about a quarter of a mile from the source, the _specus_ is open in two places, so that a man can walk along it, being nearly six feet high and three wide. Small openings have been left on the sides of the _specus_ at regular intervals to let the chalky water escape; and in falling from above it has left