If I wish to know the load which a structure will support without yielding or breaking, or its deflection under some load, I usually make calculations embodying numbers representing certain properties of the materials used, determined by some simple experiments. Sometimes the mathematics may be exceedingly difficult, or even intractable; or the mathematical solution may be achieved only at the expense of assumptions whose validity seems to need confirmation. I may then experiment with my structure, full scale; but it may be large and costly and I may wish to know the answer before I venture to build it. I may make a small scale model and experiment with that.