Farther: the midmost stone at _f_ is likely to be troublesome chiefly by its weight, pushing down between the others; the more we lighten it the better: so we will cut it into exactly the same shape as the side ones, chiselling away the shaded parts, as at _h_. We shall then have all the three stones _k_, _l_, _m_, of the same shape; and now putting them together, we have, at C, what the reader, I doubt not, will perceive at once to be a much more satisfactory arrangement than that at _f_.
§ III. We have now got three arrangements; in one using only one piece of stone, in the second two, and in the third three. The first arrangement has no particular name, except the "horizontal:" but the single stone (or beam, it may be,) is called a lintel; the second arrangement is called a "Gable;" the third an "Arch."
We might have used pieces of wood instead of stone in all these arrangements, with no difference in plan, so long as the beams were kept loose, like the stones; but as beams can be securely nailed together at the ends, we need not trouble ourselves so much about their shape or balance, and therefore the plan at _f_ is a peculiarly wooden construction (the reader will doubtless recognise in it the profile of many a farm-house roof): and again, because beams are tough, and light, and long, as compared with stones, they are admirably adapted for the constructions at A and B, the plain lintel and gable, while that at C is, for the most part, left to brick and stone.
§ IV. But farther. The constructions, A, B, and C, though very conveniently to be first considered as composed of one, two, and three pieces, are by no means necessarily so. When we have once cut the stones of the arch into a shape like that of _k_, _l_, and _m_, they will hold together, whatever their number, place, or size, as at _n_; and the great value of the arch is, that it permits small stones to be used with safety instead of large ones, which are not always to be had. Stones cut into the shape of _k_, _l_, and _m_, whether they be short or long (I have drawn them all sizes at _n_ on purpose), are called Voussoirs; this is a hard, ugly French name; but the reader will perhaps be kind enough to recollect it; it will save us both some trouble: and to make amends for this infliction, I will relieve him of the term _keystone_. One voussoir is as much a keystone as another; only people usually call the stone which is last put in the keystone; and that one happens generally to be at the top or middle of the arch.