[Illustration: HOWDEN.] At the close of this Period a feature began to make its way into the subordinate parts of the tracery, which had already shown itself for some time previously in the mouldings, and which eventually exercised a most important influence on the Architecture of the next half-century. This feature is the curve which mathematicians call the _curve of contra-flexure_, and which is known amongst architecturalists as the _Ogee_. [Illustration: WILSFORD.] The flowing nature of this curve imparted to the Tracery a grace and an ease which the rigid outline of the Circle denied it: and affords us a strong point of contrast whereby to distinguish the Architecture of the two Periods. The sinuosity of form which characterises the tracery, pervades also the mouldings, the carved work, and all the details of this Period, and enables us to designate it appropriately as the CURVILINEAR PERIOD. [Illustration: WINCHESTER.] In the latter part of this Period, a horizontal bar, or _transom_, as it is called, was occasionally used in the lower part of the window. Whether this bar was introduced for the purpose of strengthening the mullions, or for the sake of proportion, it speedily grew into frequent use. At the same time also vertical lines presented themselves occasionally in the Tracery; a new principle, in fact, had made its appearance, which rapidly overran not only the windows, but the doorways, the arcades, and every part of the building. The straight line, when once introduced, quickly superseded the curved line; square panels covered the walls; angularity of form pervaded even the mouldings and minor details, and to the round finish, the square edge was preferred. This, the last of the four Periods of Gothic Architecture which extended over a term of nearly two Centuries, we propose accordingly to call the RECTILINEAR PERIOD. The History of our National Architecture will thus be divided into Seven Periods, the order and duration of which are as follows:-- ROMANESQUE. A.D. A.D. YEARS. I. SAXON PERIOD from ---- to 1066, prevailed -- II. NORMAN PERIOD " 1066 " 1145, " 79 III. TRANSITIONAL PERIOD " 1145 " 1190, " 45 GOTHIC. IV. LANCET PERIOD " 1190 " 1245, " 55 V. GEOMETRICAL PERIOD " 1245 " 1315, " 70 VI. CURVILINEAR PERIOD " 1315 " 1360, " 45 VII. RECTILINEAR PERIOD " 1360 " 1550, " 190 [Illustration: INTERIOR COMPARTMENT.] CLERE-STORY. 27 Boss. 26 Vaulting Ribs (Transverse). 25 do. (Longitudinal). 24 Vault. 23 Sill of C. Window. 22 Mullion of do. 21 Tracery of do. 20 Arch-mouldings of C. Arch. 19 Bases of Jamb of do. 18 Capitals of do. 17 Jamb-mouldings of do. 16 Clere-story String. BLIND-STORY. (Triforium.)