351. It will be necessary to explain a little more definitely what is
meant by the strength of timber. We may conceive a rod to be broken
in three different ways. In the first place the rod may be taken by
a force at each end and torn asunder by pulling, as a thread may be
broken. To do this requires very great power, and the strength of the
material with reference to such a mode of destroying it is called its
resistance to extension. In the second place, it may be broken by
longitudinal pressure at each end, as a pillar may be crushed by the
superincumbent weight being too large; the strength that relates to
this form of force is called resistance to compression: finally, the
rod may be broken by a force applied transversely. The strength of
pine with reference to these different applications of force will be
considered successively. The rods that are to be used have been cut
from the same piece of timber, which has been selected on account of
its straightness of grain and freedom from knots. They are of different
rectangular sections, 1" × 0"·5 and 0"·5 × 0"·5 being generally used,
but sometimes 1" × 1" is employed.
[Illustration: FIG. 49.]
352. With reference to the strength of timber in its capacity to resist
extension, we can do but little in the lecture room. I have here a pine
rod A B, of dimensions 48" × 0".5 × 0"·5, Fig. 49. Each end of this
rod is firmly secured between two cheeks of iron, which are bolted
together: the rod is suspended by its upper extremity from the hook of
the epicycloidal pulley-block (Art. 213), which is itself supported by
a tripod; hooks are attached to the lower end of the rod for carrying
the weights. By placing 3 cwt. on these hooks and pulling the hand
chain of the pulley-block, I find that I can raise the weight safely,
and therefore the rod will resist at all events a tension of 3 cwt.
From experiments which have been made on the subject, it is ascertained
that about a ton would be necessary to tear such a rod asunder;
hence we see that pine is enormously strong in resisting a force of
extension. The tensile strength of the rod does not depend upon its
length, but upon the area of the cross section. That of the rod we have
used is one-fourth of a square inch, and the breaking weight of a rod
one square inch in section is about four tons.
353. A rod of any material generally elongates to some extent under
the action of a suspended weight; and we shall ascertain whether this
occurs perceptibly in wood. Before the rod was strained I had marked
two points upon it exactly 2 feet apart. When the rod supports 3 cwt.
I find that the distance between the two points has not appreciably
altered, though by more delicate measurement I have no doubt we should
find that the distance had elongated to an insignificant extent.
354. Let us contrast the resistance of a rod of timber to extension
with the effect upon a rope under the same circumstances. I have here
a rope about 0".25 diameter; it is suspended from a point, and bears a
14 lb. weight in order to be completely stretched. I mark points upon
the rope 2' apart. I now change the stone weight for a weight of 1
cwt., and on measurement I find that the two points which before were
2' apart, are now 2' 2"; thus the rope has stretched at the rate of an
inch per foot for a strain of 1 cwt., while the timber did not stretch
perceptibly for a strain of 3 cwt.
355. We have already explained in Art. 37 the meaning of the word
“tie.” The material suitable for a tie should be capable of offering
great resistance, not only to actual rupture by tension, but even to
appreciable elongation. These qualities we have found to be possessed
by wood. They are, however, possessed in a much higher degree by
wrought iron, which possesses other advantages in durability and
facility of attachment.