[Illustration: _Photo_] [_Ed. Hautecœur, Paris._ CLOCHETON OR TURRET--APSIDAL CHAPELS.] =The Buttress System.=--The buttress system of Notre Dame has been the subject of careful study and explanation by Mr. Moore. “In the external system,” he remarks, “the flying buttresses were, as at first constructed, magnificently developed, and were double in a twofold sense. That is, the piers which divide the double aisles were formerly carried up through the roof so as to form buttresses to the vaulted triforium gallery, and, rising above the roof of this gallery, they received the heads of the double flying buttresses over the outer aisle, and gave foothold to another pair of arches over the triforium gallery. The lower arch of the outer pair was above the aisle roof, while the lower arch of the inner pair was beneath the roof of the triforium. The principle of equilibrium maintained by opposing thrusts was here completely developed; the inert principle no longer governs the construction, though a survival of the former method of building is found in the walls of the aisles and clerestory, which are no longer necessary to the strength of the edifice.” The flying buttresses, as we now see them, are (according to Viollet-le-Duc) alterations dating from the early part of the thirteenth century. They consist of huge arches clearing both aisles with a single span. The flying buttresses of the upper tier are wonderfully light and elegant, looking always to the large span which they have to clear. They join the space between the windows of the clerestory to lofty upright buttresses terminating in fine crocketed pinnacles and ornamented with an amazing wealth of sculpture. The flying buttresses of the lower tier are thicker, and most frequently spring from elaborate _clochetons_, one of which is illustrated here. [Illustration: _Photo_] [_Ed. Hautecœur, Paris._ WINDOWS ON THE SOUTH SIDE.]