The Roman bridges and aqueducts in Spain are among the most justly celebrated, notably those of Segovia, Tarragona, and Alcantara. Bridging rivers by boats was a common method in use by the Roman armies under Julius Cæsar. We have also an account of a wooden bridge over the Danube, constructed by Trajan. Under every street in Rome there ran vaulted sewers conducting all impurities into the main artery, called the Cloaca Maxima, which in turn discharged its contents into the Tiber. This sewer is one of the oldest examples of the use of voussoirs, dating from the reign of Tarquinius Priscus. It is covered by a triple vault, sustaining the street above. Agrippa conducted the waters of several streams into the sewers and appointed inspectors to keep them in repair and good order. In the building of the baths of Rome, Agrippa, Nero, Vespasian, Caracalla, Titus, Diocletian, and Constantine vied with each other in the production of the most magnificent structures. They are to-day in a hopelessly ruined condition, but from the numerous fragments of carved marble and panelled stucco lying on their sites, and from the rich paintings and mosaics of the baths of Titus and Caracalla, it is not difficult to form an idea of their original splendour. It is not a little significant of what their rich decoration must have been to note that such marvels of statuary as the Laocoon, the Farnese Bull, and the Gladiators have been discovered within them. Besides the necessary administrative rooms, these buildings generally contained a frigidarium or cold bath, a tepidarium or warm bath, and a sudatorium, circular in form and covered in by a dome. The walls, built of brick, were pierced with niches and supported high cross and barrel vaults of immense span. It has been conjectured that the Pantheon was the entrance hall of the baths of Agrippa, the porch having been added at a later period when the building was converted into a temple. The chief commemorative structures were triumphal arches and votive columns. The former were of two kinds, having either one main arched opening, or a large central arch for vehicles and two lower ones on either side for foot passengers. The arch of Titus in Rome is an example of the first, its main arch being flanked by composite columns, supporting a richly carved entablature, which is in turn surmounted by an attic, inscribed with the dedication to the conqueror by the Senate and Roman people. The bassi relievi employed in its decoration represent the sacking of Jerusalem by Titus; a specially notable feature among the spoils depicted being the golden candelabra with the seven sockets, mentioned in Scripture history.