and strain was empiric up to a very late date. In 1678, Robert Hooke published his famous law of stress and deformation in materials, namely, "As the extension, so is the resistance," which he claimed to have discovered eighteen years previously and kept secret for the purpose of obtaining some patents. It is still termed Hooke's Law, but is now known to be true only within the elastic limit of any material. From that date until 1857, when Saint-Venant gave a complete analysis of the strength and elasticity of beams, engineers followed many strange hypotheses, which they dignified by styling them theories, and tried to preserve many individual secrets. Self-tutored mechanics to-day bring forth startling ideas, startling at least to modern engineers, because so many of them read reprints of books written fifty and sixty years ago. The self-tutored man should never buy a book without examining the copyright page for the date. If the copyright was obtained prior to 1895 he should not purchase the book.
When the first man wanted to cross a river without swimming and found a fallen tree spanning from bank to bank, the first bridge existed. It may have been many centuries before the human race developed to a point where it was possible to fell trees and build bridges. The bridges as late as Roman times were built of horizontal beams and girders resting on piles, with no attempt at intelligent trussing. That is, of course, wooden
