Bark (botany) rope, used for example in the rigging of Viking age longships. Among the commercial products made from bark are cork, cinnamon, quinine (from the bark of Cinchona) and aspirin (from the bark of willow trees). The bark of some trees notably oak ("Quercus robur") is a source of tannic acid, which is used in tanning. Bark chips generated as a by-product of lumber production are often used in bark mulch in western North America. Bark is important to the horticultural industry since in shredded form it is used for plants that do not thrive in ordinary soil, such as epiphytes.
Hippocrates of Chios would be known too. Only much later was it proven (by Ferdinand von Lindemann, in 1882) that this approach had no chance of success, because the factor pi (π) is transcendental. The number π is the ratio of the circumference to the diameter of a circle, and also the ratio of the area to the square of the radius. In the century after Hippocrates at least four other mathematicians wrote their own "Elements", steadily improving terminology and logical structure. In this way Hippocrates' pioneering work laid the foundation for Euclid's "Elements" (c. 325 BC) that was to remain the standard