This perpetual flow has been a characteristic of the Roman fountains since the days of the Emperors. Frontinus, writing in the reign of Trajan, says that all the great fountains were constructed with two receiving-tanks, each from a separate aqueduct, so that no accident or emergency should diminish or stop the supply of water. The later popes were also careful to preserve this uninterrupted flow, and since the close of the Cinque Cento their fountains have played unceasingly. The lowest basins of both fountains (twenty-six feet in diameter) are of travertine with a rim of Carrara marble. The middle basins (fifteen feet in diameter) are of granite. That in the right-hand fountain is of red Oriental granite, and that in the left-hand fountain of gray granite. The inverted basins at the summit, on which the water falls, are of travertine, as are also the massive shafts, which, however, Maderno adorned with a slight moulding of Carrara marble just above the water-line in the lowest basins. The entire structures have been so transformed in color by three hundred years’ deposit of the Acqua Paola that they have the appearance of bronze. The water in each fountain rises in a crowded mass of separate jets from the summit of the central and single shaft, and falls at first on an inverted basin covered by deep carving, the richness of which gains in beauty from the green web woven about its curves and angles by the fall of the water. This upper carving seems to be a part of the fantastic action of the wind-tossed spray. The lower basins which receive the water are severely plain, the design following Nature’s scheme of development, from a fretted and turbulent source to the broad surfaces of the full stream. But the architectural values of these fountains are incalculably affected by the wonderful play of the water. It leaps upward as if to meet the sun; it falls back in tumult and foam; it drenches all about with its far-flung spray and wasteful overflow. It is the very triumph of vitality and joy. The fountains of St. Peter’s might be said to bear toward the vast piazza of which they are a part the same relation as that of the eye to the human countenance: without them the noble spaces would seem cold and inanimate. This gleaming, tossing water endlessly at play with the wind and the sun, instinct with a power and a beauty not of man’s making--this it is which gives to the world-famous scene the touch of life.