The Great Pyramid and the two smaller ones near it, forming the pyramids of Gizeh, are placed on a small rocky plateau near the apex of the delta of the Nile. The largest of these is situated so that its northern face rises from the very edge of this plateau. The reason of this seems to have been that the builders wished to place it as nearly as possible on the 30th parallel of latitude. It is really about a mile and a third south of that parallel, and it is shown that such an error is a small one for that early period, and would matter but very little for the purpose required. The next feature is that it is truly oriented; that is, the four sides run north and south, east and west. It is also a true square, the four sides being of equal length, and the four corners are on a truly level plane. The first thing the builders had to do was to get a true meridian line, and they could have done this in two ways—by observations of the sun or of the pole star, the latter being much the more accurate, though more laborious and costly. At the time the pyramid was built the pole star was Alpha Draconis, which was farther from the pole than our pole star and revolved around the true pole in a circle of 7° 24′ in diameter. In order to observe the direction of this star at its lowest point, the builders excavated in the solid rock a tunnel about 4 feet in diameter, so as to keep this star visible each day at the lowest point of its circuit. This tunnel extended 350 feet through the rock to a point nearly under the centre of the pyramid, where, by a small vertical boring, a plumb-line could have been dropped so as to obtain the exact line of the meridian on the surface, and afterwards on each successive step of the pyramid as it was built up. While the building went on the sloping tunnel was continued backwards to its northern face; and a tunnel ascending to the south was formed of the same size and making the same angle with the horizon. This had puzzled all previous explorers of the pyramid till Mr. Proctor showed that, by stopping up the downward passage at the angle and filling the hollow with water the pole star could be observed by reflexion and thus give the exact direction of the meridian on the upper surface of the pyramid with extreme accuracy, as it was built up slowly year by year.