The water from the gutter behind the parapet is carried out on to the backs of the flying-buttresses by means of holes cut through the stonework. Into these pipes are passed which convey the water through to the open gutter channels of the buttresses. The backs of the raking buttresses, though they are sharply weathered to throw the water from them quickly, are also covered with lead as a further protection. These buttresses have carried the thrust of the vaults down-wards with safety for about six hundred years. But the presence of two distinct arches under each of them indicates that they have been altered a little since first they were put up. This was done when it became necessary to carry their thrust farther out because of the new chapels that were added long after the vaults were built over the nave. At the foot of each raking slope is a horizontal piece which runs out until it comes in contact with the octagon pinnacles of the vertical exterior buttresses. It should be noted that where the flying-buttresses meet the vertical wall of the clerestory there is in some cases a portion of the flat buttressing of the twelfth century visible. Between the buttresses of the chapels are four two-light windows, The outer arch of each of these windows is a beautiful example of late thirteenth-century moulded detail. The main line of the arch curve is excellent, and the whole opening between the head, jambs, and sill is beautifully proportioned. Some fifteenth century tracery remained in these windows until it was replaced by the present modern work. The outer arch is in two orders, which are carried by slight attached shafts, some of which are renewals. The capitals to these are carved, and have square abaci, rounded at the angle, as they pass over the capitals. These abaci, which are finely moulded, are not more than about two and a half inches in depth. The bases of the jamb-shafts are characteristic of the period during which this work was done. There are two small rounded mouldings, and one larger one. These rest on the square, lower part, of the base. Immediately below the sill is a string-course; and this, as well as the projecting base to the whole wall, is continued from the side of the tower buttress eastward. Each is returned round the four buttresses till it stops against the outer wall of the south walk of the cloisters. The vertical buttresses here were originally completed with a weathering at a point about half-way up their present height; and upon this old weathering the upper and later part of the buttress has been added. This was probably done during the fourteenth century, about the time that the adjoining parapet of the aisles, the parapet of the nave, and the re-working of the upper part of the flying-buttresses was undertaken. This change in the design involved the removal of the range of pointed gables, by which the roof over each bay of the aisle was completed southward.