At the time when these walls were built, the stone generally used for such purposes was the hard tufa. The greater part of the Cloaca Maxima, and the remnants of these Servian walls, are composed of this material. It is hewn into long rectangular blocks, which are placed (in builders’ phrase alternately headers and stretchers) sometimes across and sometimes along the line of the wall, in order to gain greater strength. No cement is used, but the stones are carefully fitted together and regularly shaped. It must here be observed that the rectangular shape and horizontal position of the blocks in this stonework by no means disprove its high antiquity. It is true that the so-called Pelasgian walls are built in a totally different style, for the stones in them are polygonal. But this difference of shape in the stones arises from a difference in the material. All the so-called Pelasgian walls in Italy are built of a stone which naturally breaks into polygonal masses; but tufa stone is found in the quarry in horizontal layers, and is most easily cut into a rectangular shape. The inference sometimes drawn from horizontally laid masonry that it indicates a more advanced state of art than polygonal cannot be relied upon as certain. The arch in this ruin is of a later date and may have been an embrasure for a catapult.[114] [Sidenote: Porta Capena.] The situation of no gate in the Servian Avails can be determined so completely as that of the Porta Capena. We know that part of the Aqua Marcia passed over it, whence it was called the dripping gate (Madida Capena) by Martial and Juvenal. It was therefore in the valley below the Cælian Hill, and we should, judging from the form of the ground, naturally place it where the hill, on which S. Balbina stands, approaches the Cælian most nearly. A striking confirmation of this conjecture has been discovered. The first milestone on the Appian Road was found in 1584 in the first vineyard beyond the present Porta S. Sebastiano--the Vigna Naro--and, measuring back one mile from it, we come exactly to this spot. This milestone is now placed on the steps leading up to the Capitoline Museum. Milestones and horse-blocks were erected on all the great roads by Caius Gracchus before the milliarium aureum was put up in the Forum by Augustus, and it is probable that the distances were always measured from the gates. Mr. Parker carried on excavations for some time to find the exact position of the Porta Capena, and he discovered some of the piers of the aqueduct which passed over the gate in the garden of the Convent of S. Gregorio. These excavations have unfortunately been more or less filled up again. [Sidenote: Monte Testaccio.]