How far the grand gallery corresponds with these requirements can be judged from the following description given by Professor Greaves in 1638:--"It is," he says, "a very stately piece of work, and not inferior, either in respect of the curiosity of art, or richness of materials, to the most sumptuous and magnificent buildings," and a little further on he says, "this gallery, or corridor, or whatever else I may call it, is built of white and polished marble (limestone), the which is very evenly cut in spacious squares or tables. Of such materials as is the pavement, such is the roof and such are the side walls that flank it; the coagmentation or knitting of the joints is so close, that they are scarce discernible to a curious eye; and that which adds grace to the whole structure, though it makes the passage the more slippery and difficult, is the acclivity or rising of the ascent. The height of this gallery is 26 feet" (Professor Smyth's careful measurements show the true height to be more nearly 28 feet), "the breadth of 6.870 feet, of which 3.435 feet are to be allowed for the way in the midst, which is set and bounded on both sides with two banks (like benches) of sleek and polished stone; each of these hath 1.717 of a foot in breadth, and as much in depth." These measurements are not strictly exact. Smyth made the breadth of the gallery above the banks or ramps as he calls them, 6 feet 10-1/5 inches; the space between the ramps, 3 feet 6 inches; the ramps nearly about 1 foot 8-1/14 inches broad, and nearly 1 foot 9 inches high, measured transversely, that is at right angles to the ascending floor.
As to arrangements for the convenience of observers in the slippery and difficult floor of this gallery, we find that upon the top of these benches or ramps, near the angle where they meet the wall, "there are little spaces cut in right-angled parallel figures, set on each side opposite one another, _intended no question for some other end than ornament_."
The diversity of width which I have indicated as a desirable feature in a meridional gallery, is a marked feature of the actual gallery. "In the casting and ranging of the marbles" (limestone), "in both the side walls, there is one piece of architecture," says Greaves, "in my judgment very graceful, and that is that all the courses or stones, which are but seven (so great are these stones), do set and flag over one another about three inches; the bottom of the uppermost course overlapping the top of the next, and so in order, the rest as they descend." The faces of these stones are exactly vertical, and as the width of the gallery diminishes upwards by about six inches for each successive course, it follows that the width at the top is about 3-1/2 feet less than the width, 6 feet 10-1/5 inches, at the bottom, or agrees in fact with the width of the space between the benches or ramps. Thus the shadow of the vertical edges of the gallery at solar noon just reached to the edges of the ramps, the shadow of the next lower vertical edges falling three inches from the edges higher up the ramps, those of the next vertical edges six inches from these edges, still higher up, and so forth. The true hour of the sun's southing could thus be most accurately determined by seven sets of observers placed in different parts of the gallery, and near mid-summer, when the range of the shadows would be so far shortened, that a smaller number of observers only could follow the shadows' motions; but in some respects, the observations in this part of the year could be more readily and exactly made than in winter, when the shadows' spaces of various width would range along the entire length of the gallery.