t the
efficacy of the fir, nor the cypress that of the elm. Nor in the case of
other trees, is it natural that they should be alike; but the individual
kinds are effective in building, some in one way, some in another, owing
to the different properties of their elements.

6. To begin with fir: it contains a great deal of air and fire with very
little moisture and the earthy, so that, as its natural properties are
of the lighter class, it is not heavy. Hence, its consistence being
naturally stiff, it does not easily bend under the load, and keeps its
straightness when used in the framework. But it contains so much heat
that it generates and encourages decay, which spoils it; and it also
kindles fire quickly because of the air in its body, which is so open
that it takes in fire and so gives out a great flame.

7. The part which is nearest to the earth before the tree is cut down
takes up moisture through the roots from the immediate neighbourhood and
hence is without knots and is "clear." But the upper part, on account of
the great heat in it, throws up branches into the air through the knots;
and this, when it is cut off about twenty feet from the ground and then
hewn, is called "knotwood" because of its hardness and knottiness. The
lowest part, after the tree is cut down and the sapwood of the same
thrown away, is split up into four pieces and prepared for joiner's
work, and so is called "clearstock."

8. Oak, on the other hand, having enough and to spare of the earthy
among its elements, and containing but little moisture, air, and fire,
lasts for an unlimited period when buried in underground structures. It
follows that when exposed to moisture, as its texture is not loose and
porous, it cannot take in liquid on account of its compactness, but,
withdrawing from the moisture, it resists it and warps, thus making
cracks in the structures in which it is used.

9. The winter oak, being composed of a moderate amount of all the
elements, is very useful in buildings, but when in a moist place, it
takes in water to its centre through its pores, its air and fire being
expelled by the influence of the moisture, and so it rots. The Turkey
oak and the beech, both containing a mixture of moisture, fire, and the
earthy, with a great deal of air, through this loose texture take in
moisture to their centre and soon decay. White and black poplar, as well
as willow, linden, and the agnus castus, containing an abundance of
fire and