The remains with which we are acquainted indicate four distinct periods of great architectural activity in Egyptian history, viz.: (1) the period of the fourth dynasty, when the Great Pyramids were erected (probably 3500 to 3000 B.C.); (2) the period of the twelfth dynasty, to which belong the remains at Beni-Hassan; (3) the period of the eighteenth and nineteenth dynasties, when Thebes was in its glory, which is attested by the ruins of Luxor and Karnak; and (4) the Ptolemaic period, of which there are the remains at Denderah, Edfou, and Philæ. The monuments that remain are almost exclusively tombs and temples. The tombs are, generally speaking, all met with on the east or right bank of the Nile: among them must be classed those grandest and oldest monuments of Egyptian skill, the Pyramids, which appear to have been all designed as royal burying-places. A large number of pyramids have been discovered, but those of Gizeh, near Cairo, are the largest and the best known, and also probably the oldest which can be authenticated.[1] The three largest pyramids are those of Cheops, Cephren, and Mycerinus at Gizeh (or, as the names are more correctly written, Suphis, Sensuphis, and Moscheris or Mencheris). These monarchs all belonged to the fourth dynasty, and the most probable date to be assigned to them is about 3000 B.C. The pyramid of Suphis is the largest, and is the one familiarly known as the Great Pyramid; it has a square base, the side of which is 760 feet long,[2] a height of 484 feet, and an area of 577,600 square feet. In this pyramid the angle of inclination of the sloping sides to the base is 51° 51', but in no two pyramids is this angle the same. There can be no doubt that these huge monuments were erected each as the tomb of an individual king, whose efforts were directed towards making it everlasting, and the greatest pains were taken to render the access to the burial chamber extremely hard to discover. This accounts for the vast disproportion between the lavish amount of material used for the pyramid and the smallness of the cavity enclosed in it (Fig. 8). The material employed was limestone cased with syenite (granite from Syene), and the internal passages were lined with granite. The granite of the casing has entirely disappeared, but that employed as linings is still in its place, and so skilfully worked that it would not be possible to introduce even a sheet of paper between the joints. [Illustration: FIG. 8.--SECTION ACROSS THE GREAT PYRAMID (OF CHEOPS OR SUPHIS).]