346. The uses of timber in the arts are as various as its qualities.
Some woods are useful for their beauty, and others for their strength
or durability under different circumstances. We shall only employ
“pine” in our experimental inquiries. This wood is selected because it
is so well known and so much used. A knowledge of the properties of
pine would probably be more useful than a knowledge of the properties
of any other wood, and at the same time it must be remembered that
the laws which we shall establish by means of slips of pine may be
generally applied.
347. A transverse section of a tree shows a number of rings, each
of which represents the growth of wood in one year. The age of the
tree may sometimes be approximately found by counting the number of
distinguishable rings. The outer rings are the newer portions of the
wood.
348. When a tree is felled it contains a large quantity of sap, which
must be allowed to evaporate before the wood is fit for use. With this
object the timber is stored in suitable yards for two or more years
according to the purposes for which it is intended; sometimes the
process of seasoning, as it is called, is hastened by other means.
Wood, when seasoning, contracts; hence blocks of timber are often
found split from the circumference to the centre, for the outer rings,
being newer and containing more sap, contract more than the inner
rings. For the same reason a plank is found to warp when the wood is
not thoroughly seasoned. The side of the plank which was farthest from
the centre of the tree contracts more than the other side, and becomes
concave. This can be easily verified by looking at the edge of the
plank, for we there see the rings of which it is composed.
349. Timber may be softened by steaming. I have here a rod of pine,
24" × 0"·5 × 0"·5, and here a second rod cut from the same piece and
of the same size, which has been exposed to steam of boiling water for
more than an hour: securing these at one end to a firm stand, I bend
them down together, and you see that after the dry rod has broken the
steamed rod can be bent much farther before it gives way. This property
of wood is utilized in shaping the timbers of wooden ships. We shall
be able to understand the action of steam if we reflect that wood is
composed of a number of fibres ranged side by side and united together.
A rope is composed of a number of fibres laid together and twisted, but
the fibres are not coherent as they are in wood. Hence we find that a
rod of wood is stiff, while a rope is flexible. The steam finds its way
into the interstices between the fibres of the wood; it softens their
connections, and increases the pliability of the fibres themselves, and
thus, the operation of steaming tends to soften a piece of timber and
render it tractable.
350. The structure of wood is exhibited by the following simple
experiment:—Here are two pieces of pine, each 9" × 1" × 1". One of
them I easily snap across with a blow, while my blows are unable to
break the other. The difference is merely that one of these pieces is
cut against the grain, while the other is with it. In the first case
I have only to separate the connection between the fibres, which is
quite easy. In the other case I would have to tear asunder the fibres
themselves, which is vastly more difficult. To a certain extent the
grained structure is also found in wrought iron, but the contrast
between the strength of iron with the grain and against the grain is
not so marked as it is in wood.