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3If hills intervene between the city walls and the spring head, tunnels under ground must be made preserving the fall above assigned; if the ground cut through be sandstone or stone, the channel may be cut therein, but if the soil be earth or gravel, side walls must be built, and an arch turned over, and through this the water may be conducted. The distance between the shafts over the tunnelled part is to be one hundred and twenty feet.
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