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.