166 THE STONES OF VENICE CONSTRUCTION

thicken your shell, but need you thicken it all equally? Not so; you would only waste your good voussoirs. If you have any common sense you will thicken it at the top, where a Mylodon’s skull¹ is thickened for the same purpose (and some human skulls, I fancy), as at b. The pebbles and gravel above will now shoot off it right and left, as the bullets do off a cuirassier’s breastplate, and will have no chance of beating it in.²

If still it be not strong enough, a further addition may be made, as at c, now thickening the voussoirs a little at the base also. But as this may perhaps throw the arch inconveniently high, or occasion a waste of voussoirs at the top, we may employ another expedient.

§ 4. I imagine the reader’s common sense, if not his previous knowledge, will enable him to understand that if the arch at a, Plate 3, burst in at the top, it must burst out at the sides. Set up two pieces of pasteboard, edge to edge, and press them down with your hand, and you will see them bend out at the sides. Therefore, if you can keep the arch from starting out at the points p, p, it cannot curve in at the top, put what weight on it you will, unless by sheer crushing of the stones to fragments.

§ 5. Now you may keep the arch from starting out at p by loading it at p, putting more weight upon it and against it at that point; and this, in practice, is the way it is usually done. But we assume at present that the weight above is sand or water, quite unmanageable, not to be directed to the points we choose; and in practice, it may sometimes happen that we cannot put weight upon the arch at p. We may perhaps want an opening above it, or it may be at the side

¹ [For the double skull of the mylodon, see Seven Lamps, ch. ii. § 13, Vol. VIII. p. 72.]

² [The MS. here gives a footnote:-

“I give this simple reason for adopting the form, for the sake of the general reader; and when there is one good reason for doing a thing and no reason against it, it is wasted time to look for another on its side. But the architect will see in a moment that the principal value of the form consists in the deeper inclination given to the line of resistance of the voussoirs.”]

[Version 0.04: March 2008]