One of the most ancient arches as yet discovered is that which was brought to light during the course of the excavations carried on by Peters, Harper, Haynes and Hilprecht at the ancient city of Nippur (cf. Fig. 15). It was found at a great depth below the surface of the mound, being more than twenty-two and a half feet below the pavement of Ur-Engur (_circ._ 2400 B.C.), and fourteen feet below that of Narâm-Sin (_circ._ 2700 B.C.); it is a true keystone arch pointed in shape, made of well-burnt plano-convex bricks, and measuring a little over two feet in height and having a span of about one foot eight inches, while its length is about three feet, but it seems probable that originally the tunnel was vaulted throughout. The irregularity of its construction somewhat diminishes the significance that it would otherwise have, but it is of supreme interest as testifying to the fact that the principle of the arch was known at this very remote period, however crude the embodiment of that principle may happen to be. The plano-convex bricks composing this arch measure 12 × 6 × 2-1/2 inches and bear the impress of finger-marks on their convex side, a characteristic feature of pre-Sargonic bricks at Nippur, Tellô and elsewhere, while the clay from which the bricks are made is of a light yellow colour. The tunnel itself seems to have been “a protecting structure for a drain,”[83] rather than a drain itself, for below the pavement two terra-cotta pipes were discovered, the existence of which can only be explained on this hypothesis. At the top of the arch were found the remains of another terra-cotta pipe, the object of which must have been to drain off the percolating rain-water, and thus prevent it penetrating through and disintegrating the vaulted structure below. The T-shaped centre-piece, which was similarly made of plano-convex bricks, doubtless served the purpose of keeping the sides of the arch from falling in. Haynes further informs us that in one of the private houses at Nippur which had been occupied at least three times, the earliest of the three doorways traceable in the ruins, consisted in a segmental arch. Another very early arch was discovered by M. De Sarzec at Tellô, close to the building of Ur-Ninâ (cf. Fig. 16), having much the same shape as the Nippur arch illustrated in Fig. 15 and doubtless used for a similar purpose, while vaulted passages of which the arch was semicircular, were discovered by Taylor[84] in his excavations at Muḳeyyer (Ur), as early as 1855.