The great discovery of this class is that of the city gates at Khorsabad, which, as mentioned at p. 181, were spanned by arches of semicircular form, so perfect both in construction and in the mode in which they were ornamented, as to prove that in the time of Sargon the arch was a usual and well-understood building expedient, and one consequently which we may fairly assume to have been long in use. [Illustration: 103. Arch at Dêr-el-Bahree. (Lepsius.)] On the other hand, we have in the temple at Dêr-el-Bahree in Thebes, built by Thothmes III., a curious example of the retention of the old form, when at first sight it would appear as though the true arch would have been a more correct expedient. In this example, the lower arch is composed of stones bracketing forward horizontally, though the form of the arch is semicircular; and above this is a discharging arch of two stones used as in the Pyramids. The upper arch is so arranged as to relieve the crown of the lower—which is its weakest part—of all weight, and at the same time to throw the whole pressure on the outer ends of the arch stones, exactly where it is wanted. The whole thus becomes constructively perfect, though it is a more expensive way of attaining the end desired than by an arch. The truth seems to be, the Egyptians had not at this age invented voussoirs deeper in the direction of the radii of the arch than in that of its perimeter; and the arch with them was consequently not generally an appropriate mode of roofing. It was the Romans with their tiles who first really understood the true employment of the arch. So far as we can now understand from the discoveries that have been made, it seems that the Assyrians used the pointed arch for tunnels, aqueducts, and generally for underground work where they feared great superincumbent pressure on the apex, and the round arch above-ground where that was not to be dreaded; and in this they probably showed more science and discrimination than we do in such works. [Illustration: 104. Arch of the Cloaca Maxima, Rome. Scale 50 ft. to 1 in.] In Europe the oldest arch is probably that of Cloaca Maxima at Rome, constructed under the early kings. It is of stone in 3 rims, and shows as perfect a knowledge of the principle as any subsequent example. Its lasting uninjured to the present day proves how well the art was then understood, and, by inference, how long it must have been practised before reaching that degree of perfection. From all this it becomes almost certain that the arch was used as early as the times of the pyramid-builders of the 4th dynasty, and was copied in the tombs of Beni-Hasan in the 12th; though it may be that the earliest existing example cannot be dated further back than the first kings of the 18th dynasty; from that time, however, there can be no doubt that it was currently used, not only in Egypt, but also in Ethiopia and Assyria.