The direction of this channel, after it enters Rome, happens to be easily seen above, as it is marked throughout its course, from the eastern end of the Cœlian to the Arch of Dolabella, by the fine arches of the Neronian branch from the Claudian Aqueduct erected almost over it. It is very probable that one of the laws (referred to by Frontinus as having been in existence long before his time) respecting aqueducts was in force from the commencement, namely, that there should be no buildings of any kind within ten feet on either side of the aqueducts; thus an open space was probably existing at the time when Nero wished to carry water to his magnificent reservoir at the north-western end of the Cœlian, and of this he availed himself in rearing the splendid arcade which is called by his name. It will be remarked, however, that the brick buttresses actually came down to the flat roof of the original _specus_ of the Appian. The top of that _specus_ was raised to a greater height by the engineers of Sixtus V., and in this enlargement the underground supports of the Neronian arches are cut through to give headway to the _aquarii_ passing along the channel. They have however to stoop their heads as they pass under each buttress. The old tunnel _specus_ was discovered in the time of Sixtus V., (Felice Peretti), filled up by clay deposit to the depth of three feet, or half the height of the _specus_, as in other places, and the builders found it easier to raise it, for the _aquarii_ to go along it, than to clear it out. They therefore knocked away the flat tile-covering of the time of Nero, and raised a vault of rubble-stone three feet higher.