This disposition is established, because by the tax on the water carried to private houses, the State keeps the

aqueduct in repair.

But, should mountains intervene between the city and the spring head, a subterranean passage is to be dug through the earth, having the declivity of one part in two hundred, and should the soil be either gravel or stone, a channel is to be cut into it, but walls are to be built to conduct the water through the earthy or sandy soils. Wells also, or air-holes are to be cut from the top of the water-channel to the surface, for the

purpose of allowing the air .which might accumulate in the aqueduct to escape. These wells are directed to be placed at a distance of 120 feet. If the water be conducted by leaden pipes, a castellum or reservoir is first built at

the spring head, then the diameter and strength of the pipes being suited to the quantity of water, they are to be carried from the castellum to that which is in the city. The

pipes are not to be less than ten Roman feet in length, and were named from the breadth of the lead before it was rounded into a pipe.

The manner of conducting water by pipes is thus regulated ; if the spring head had

a sufficient current to the city, and no higher hills intervened, the interval is, by

walling, raised to a proper level, as mentioned in the description of channels of masonry, or else a circuit round may be taken if not very long but if there be frequent ;