Further, from the great chamber are carried two ventilation-shafts, or air-passages, northwards and southwards, which open on the outer surface of the pyramid, and are respectively two hundred and thirty-three and one hundred and ninety-four feet long. These passages are square, or nearly so, and have a diameter varying between six and nine inches. They give a continual supply of pure air to the chamber, and keep it dry at all seasons. The Great Gallery is also of curious construction. Extending for a distance of one hundred and fifty feet, and rising at an angle of 26° 18', it has a width of five feet at the base and a height of above thirty feet. The side walls are formed of seven layers of stone, each projecting a few inches over that below it. The gallery thus gradually contracts towards the top, which has a width of four feet only, and is covered in with stones that reach across it, and rest on the walls at either side. The exact object of so lofty a gallery has not been ascertained; but it must have helped to keep the air of the interior pure and sweet, by increasing the space through which it had to circulate. The "Pyramid Builders," or kings who constructed the three monuments that have now been described, were, according to a unanimous tradition, three consecutive monarchs, whose native names are read as Khufu, Shafra, and Menkaura. These kings belonged to Manetho's fourth dynasty; and Khufu, the first of the three, seems to have been the immediate successor of Sneferu. Theorists have delighted to indulge in speculations as to the objects which the builders had in view when they raised such magnificent constructions. One holds that the Great Pyramid, at any rate, was built to embody cosmic discoveries, as the exact length of the earth's diameter and circumference, the length of an arc of the meridian, and the true unit of measure. Another believes the great work of Khufu to have been an observatory, and the ventilating passages to have been designed for "telescopes," through which observations were to be made upon the sun and stars; but it has not yet been shown that there is any valid foundation for these fancies, which have been spun with much art out of the delicate fabric of their propounders' brains. The one hard fact which rests upon abundant evidence is this--the pyramids were built for tombs, to contain the mummies of deceased Egyptians. The chambers in their interiors, at the time of their discovery, held within them sarcophagi, and in one instance the sarcophagus had within it a coffin. The coffin had an inscription upon it, which showed that it had once contained the body of a king. If anything more is necessary, we may add that every pyramid in Egypt--and there are, as he have said, more than sixty of them--was built for the same purpose, and that they all occupy sites in the great necropolis, or burial-ground opposite Memphis, where the inhabitants are known to have laid their dead.