76 THE STONES OF VENICE

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.

§ 5. (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 is called an arch; but a flat 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. 1. Having done this, let him imagine these several divisions, first moved along (or set side by side) over a rectangle, b, Fig. 1, and then revolved round

[Version 0.04: March 2008]