158 THE STONES OF VENICE CONSTRUCTION

One is the visible direction or curve of the arch, which may generally be considered as the under edge of its voussoirs, and which has often no more to do with the real stability of the arch, than a man’s apparent conduct has with his heart. The other line, which is the line of resistance, or line of good behaviour, may or may not be consistent with the outward and apparent curves of the arch; but if not, then the security of the arch depends simply upon this, whether the voussoirs which assume or pretend to the one line are wide enough to include the other.

§ 9. Now when the reader is told that the line of resistance varies with every change either in place or quantity of the weight above the arch, he will see at once that we have no chance of arranging arches by their moral characters: we can only take the apparent arch line, or visible direction, as a ground of arrangement. We shall consider the possible or probable forms or contours of arches in the present Chapter, and in the succeeding one the forms of voussoir and other help which may best fortify these visible lines against every temptation to lose their consistency.

§ 10. Look back to Fig. 29. Evidently the abstract or ghost line of the arrangement at A is a plain horizontal line, as here at a, Fig. 30. The abstract line of the arrangement at B, Fig. 29, is composed of two straight lines set against each other, as here at b. The abstract line of C, Fig. 29, is a curve of some kind, not at present determined, suppose c, Fig. 30. Then, as b is two of the straight lines at a, set up against each other, we may conceive an arrangement, d, made up of two of the curved lines at c, set against each other. This is called a pointed arch, which is a contradiction in terms: it ought to be called a curved gable; but it must keep the name it has got.

Now, a, b, c, d, Fig. 30, are the ghosts of the lintel, the gable, the arch, and the pointed arch. With the poor lintel ghost we need trouble ourselves no farther; there are no changes in him: but there is much variety in the other three, and the method of their variety will be best discerned by

[Version 0.04: March 2008]