If its functions of partition or enclosure are continued, together with that of resisting vertical pressure, it remains as a wall veil between the piers into which it has been partly gathered; but if it is required only to resist the vertical or roof pressure, it is gathered up into piers altogether, loses its wall character, and becomes a group or line of piers. On the other hand, if the lateral pressure be slight, it may retain its character of a wall, being supported against the pressure by buttresses at intervals; but if the lateral pressure be very great, it is supported against such pressure by a continuous buttress, loses its wall character, and becomes a dyke or rampart. § IV. We shall have therefore (A) first to get a general idea of a wall, and of right construction of walls; then (B) to see how this wall is gathered into piers; and to get a general idea of piers and the right construction of piers; then (C) to see how a wall is supported by buttresses, and to get a general idea of buttresses and the right construction of buttresses. This is surely very simple, and it is all we shall have to do with walls and their divisions. [Illustration: Fig. I.] § V. 2. _Roofs._--A roof is the covering of a space, narrow or wide. It will be most conveniently studied by first considering the forms in which it may be carried over a narrow space, and then expanding these on a wide plan; only there is some difficulty here in the nomenclature, for an arched roof over a narrow space has (I believe) no name, except that which belongs properly to the piece of stone or wood composing such a roof, namely, lintel. But the reader will have no difficulty in understanding that he is first to consider roofs on the section only, thinking how best to construct a narrow bar or slice of them, of whatever form; as, for instance, _x_, _y_, or _z_, over the plan or area _a_, Fig. I. Having done this, let him imagine these several divisions, first moved along (or set side by side) over a rectangle, _b_, Fig. I., and then revolved round a point (or crossed at it) over a polygon, _c_, or circle, _d_, and he will have every form of simple roof: the arched section giving successively the vaulted roof and dome, and the gabled section giving the gabled roof and spire. As we go farther into the subject, we shall only have to add one or two forms to the sections here given, in order to embrace all the _uncombined_ roofs in existence; and we shall not trouble the reader with many questions respecting cross-vaulting, and other modes of their combination.