But at a distance of 127 feet a new feature appears. The ascending tunnel is changed into what is called the Great Gallery, which, while continuing exactly the same floor line as the tunnel, is suddenly raised to a height of 28 feet, with a width of 7 feet on the floor and 3½ feet at the top. Along each side there is a ledge or seat, 20 inches broad and 21 inches high. The sides do not slope inwards, but are formed of seven courses of stone, each one overlapping the one below by about 3 inches. The whole of this gallery, or inclined corridor, is formed of limestone beautifully smooth, or even polished. The length of this gallery is 156 feet, and its floor terminated at the platform of the pyramid, upon the central line from east to west, when it had reached two-thirds of its total height. This is on the level of the King's Chamber; and it was probably only after the king was dead and his body embalmed and placed in his sarcophagus that the pyramid was completed, the openings of the passages carefully closed up, and the whole exterior covered with a smooth casing of stone, very small portions of which now remain. There are two other features of this gallery which have puzzled the merely antiquarian explorers. These are square holes cut in the sloping benches close to the side walls, and about 5½ feet apart, there being eighteen on each side exactly opposite each other. On each side of the gallery, about half-way up, is a longitudinal groove, which would serve to carry transverse screens which could be slid up or down, and easily wedged in position in order to mark exactly the central line, like the cross hairs in an astronomical telescope. The holes on the benches would serve to carry cross seats on which the observer could be firmly and comfortably seated while observing a transit of sun, star, or planet.
Being open to the south, the Great Gallery would give a magnificent view of the southern sky, and enable observers to determine the altitudes and azimuths of many stars, and of the superior planets Mars, Jupiter and Saturn. The star Alpha Centauri, which was at that period of the first magnitude though now much diminished in brightness, would, when crossing the meridian, have been situated about the centre of the field of view as seen from this remarkable feature of the pyramid which, Mr. Proctor considers, was the finest transit-instrument ever constructed for naked-eye observations. Tycho Brahé, with his celebrated Quadrant at Uranienburg, did not attain such a degree of accuracy as did these Eastern astronomers nearly 6,000 years ago. One great superiority of the subterranean observatory over any open-air observations that can be made without telescopes is, that by closing up the end, except for the small aperture required to see the object, the brighter stars could be well observed in the daytime.