172
HISTORY OF ENGINEERING.
Book I.
'Die castellum of Aqua Julia is situated near the Porta Esquilina ; in it we can trace thetriple immissarium from which the waters were distributed. The castellum (Vitruvius ,lib. viii. cap. 7.) or reservoir, constructed near the walls of the city, had a triple cisternattached to it to receive the water. Three conduits, of equal dimensions, were connectedin such a manner, that when the water was more than necessary for the supply of the outer,
CASTELLUM AQUA JULIA.
it was discharged into that of the middle, which served all the pipes of the public foun-tains: one of the mains supplied the baths, the other the private houses. The object ofthis contrivance was to provide first for the public wants, then the baths, and afterwardsprivate individuals.
At the end of each of these three conduits was a receptacle (receptaculum), from whencethe general distribution was made ; at the sides were two others (caduca?) to take off anysuperabundant quantity. By such an arrangement the various supplies were regulatedwith the greatest nicety. The total width of this castellum is 115 feet; and the plan andsections show its general arrangement, its staircases, passages, and receptacles for the water.No expense was spared in the construction of these stupendous edifices, which, attached tothe numerous aqueducts of Rome, must have resembled palaces. Built of squared stone,and lined with brick coated with a fine cement, every precaution was taken to preventleakage or infiltration. The several conduits and pipes were provided with valves andcorks for shutting off* or turning on a supply to any direction. Here the superintendautcould direct the flow of water to the several localities in his neighbourhood, without goinginto the castellum: he could also judge of the quantity discharged in each direction, bythe simple instruments he was provided with : experience soon taught him what quantityflowed through the respective apertures, and he always knew, by guaging the several cisternsor reservoirs, what had flowed out. A constant flow from the aqueduct enabled him atthe castellum to regulate its distribution, and without great arrangement, when the timecame for supplying the numerous baths, there would have been found a deficiency: thereneeded some precautions to prevent this.
As the waters brought by these several conduits deposited a considerable quantity ofearthy matter, it was necessary that the castellum which received it should be providedwith conveniences, by which all the silt that accumulated could be readily removed. Forthis purpose chambers were attached to the several cisterns, where the water was not dis-turbed by the efflux or letting it out, and from thqm the deposit was washed, by eitherletting it off into the public sewers, or removing it by manual labour. In some of thecisterns or reservoirs discovered, the bottom was made with a considerable fall or inclination