CONSTRUCTION X. THE ARCH LINE 157

houses, and so also the gable: there are, therefore, two distinct questions respecting each arrangement;-First, what is the line, or direction of it, which gives it its strength? and, secondly, what is the manner of masonry of it, which gives it its consistence? The first of these I shall consider in this Chapter under the head of the Arch Line, using the term arch as including all manner of construction (though we shall have no trouble except about curves); and in the next Chapter I shall consider the second, under the head, Arch Masonry.

§ 6. Now the arch line is the ghost or skeleton of the arch; or rather it is the spinal marrow of the arch, and the voussoirs are the vertebræ, which keep it safe and sound, and clothe it. This arch line the architect has first to conceive and shape in his mind, as opposed to, or having to bear, certain forces which will try to distort it this way and that; and against which he is first to direct and bend the line itself into as strong resistance as he may, and then, with his voussoirs and what else he can, to guard it, and help it, and keep it to its duty and in its shape. So the arch line is the moral character of the arch, and the adverse forces are its temptations; and the voussoirs, and what else we may help it with, are its armour and its motives to good conduct.

§ 7. This moral character of the arch is called by architects its “Line of Resistance.” There is a great deal of nicety in calculating it with precision, just as there is sometimes in finding out very precisely what is a man’s true line of moral conduct: but this, in arch morality and in man morality, is a very simple and easily to be understood principle,-that if either arch or man expose themselves to their special temptations or adverse forces, outside of their voussoirs or proper and appointed armour, both will fall. An arch whose line of resistance is in the middle of its voussoirs is perfectly safe: in proportion as the said line runs near the edge of its voussoirs, the arch is in danger, as the man is who nears temptation; and the moment the line of resistance emerges out of the voussoirs the arch falls.

§ 8. There are, therefore, properly speaking, two arch lines.

[Version 0.04: March 2008]