415. In this lecture and the next we shall experiment upon some of the
arts of construction. We shall employ slips of pine 0"·5 × 0"·5 in
section for the purpose of making models of simple framework: these
slips can be attached to each other by means of the small clamps about
3" long, shown in Fig. 56, and the general appearance of the models
thus produced may be seen from Figs. 58 and 62.
[Illustration: FIG. 56.]
416. The following experiment shows the tenacity with which these
clamps hold. Two slips of pine, each 12" × 0"·5 × 0"·5, are clamped
together, so that they overlap about 2", thus forming a length of
22": this composite rod is raised by a pulley-block as in Fig. 49,
while a load of 2 cwt. is suspended from it. Thus the clamped rods
bear a direct tension of 2 cwt. The efficiency of the clamps depends
principally upon friction, aided doubtless by a slight crushing of the
wood, which brings the surfaces into perfect contact.
417. These slips of pine united by the clamps are possessed of strength
quite sufficient for the experiments now to be described. Models thus
constructed have the great advantage of being erected, varied or pulled
down, with the utmost facility.
We have learned that the compressive strength, and, still more, the
tensile strength of timber, is much greater than its transverse
strength. This principle is largely used in the arts of construction.
We endeavour by means of suitable combinations to turn transverse
forces into forces of tension or compression, and thus strengthen our
constructions. We shall illustrate the mode of doing so by simple forms
of framework.