The Romans were lavish users of water, for purposes of luxury as well as necessity. They understood the simple hydraulic law that water will rise in pipes to its own original level and applied the system in their buildings. But since pipes of lead and bronze were costly and none too durable, they dispensed as far as possible with their use, conveying the water in lofty aqueducts, with a fall, as Vitruvius recommended, of 6 inches in 100 feet, so that the water was delivered from a height at the spot it was needed. The channel, constructed of concrete, lined with cement, was conducted upon a series of concrete arches, faced with brick; the arches being of immense height and sometimes in several tiers. The =Anio Novus=, constructed <small>A.D.</small> 38, was sixty-two miles in length and entered =Rome= on arches carried over the =Aqua Claudia=, which was erected at the same time and is still one of the water supplies of Rome. The finest existing example, however, is the so-called =Pont-du-Gard=, near =Nîmes=, which forms part of an aqueduct twenty-five miles long. For a distance of about 900 feet it is composed of three tiers of arches, crossing the valley 180 feet above the River Gard. * * * * * In conclusion, the genius of the Roman architect consisted in his faculty of organisation, which enabled him to take the principles of Hellenic architecture and apply them to a great variety of requirements. What his architecture lost in refinement, it more than gained in flexibility and resourcefulness, while creating for itself a distinction of structural grandeur. It refertilised the Hellenic which had threatened to become a barren style and produced a style that not only was richly competent to serve the needs of its own time, but has proved capable of being further developed to new needs. It involved principles that had their influence on Romanesque and consequently on Gothic architecture, became the source from which Renaissance architecture was evolved, and, even in our own day, are still capable of new and active service. BOOK IV POST-CLASSIC PERIOD CHAPTER I EARLY CHRISTIAN CIVILISATION As the power of Rome waned and the Empire became disintegrated, the force of Christianity increased and spread and the organisation of the Church became consolidated. The immediate followers of Christ looked for their Lord’s reappearance as a Jewish Messiah. Paul, however, taught that there was no distinction in the sight of Christ between Jew and Gentile and treated Christianity as a philosophic system of ethics, applicable to all races and conditions of rich and poor. His view prevailed and Christianity became a great proselytising force.