Traditional units of volume in British forestry. In a 1736 manual of practical calculation, the English surveyor Edward Hoppus advised foresters to estimate the volume of wood in a log of length L and girth (circumference) G as L·(G/4)². Since the actual volume of a cylinder is L·G²/(4·pi), the resulting figure, called the hoppus volume, is smaller than the actual volume of the log. In fact not all the wood in a log can be used, and the hoppus volume was considered a fairly reasonable estimate of the usable volume of wood in the log. Volume measurements made using the hoppus formula are stated in hoppus feet. In effect, this makes the hoppus foot a unit of volume equal to 4/pi = 1.273 cubic feet or 0.036 054 cubic meter. Similarly, the hoppus board foot is equal to 1/12 hoppus foot or 1.273 board feet, which is almost exactly 3 liters (0.00300 cubic meter). The British forestry industry changed its unit of timber measurement from hoppus feet to cubic meters in 1971