GRAIN DRILL

machine for planting seed at a controlled depth and in specified amounts. The earliest known version, invented in Mesopotamia by 2000 BC, consisted of a wooden plow equipped with a seed hopper and a tube that conveyed the seed to the furrow. By the 17th century, metering systems were in use to ensure accuracy of the rate of planting; most consisted of wheels bearing small spoons that dipped into the seed hopper and guided it to the furrows in standard amounts. Modern grain drills have a variety of metering systems and furrow openers. In general, the metering device-spoon, cup, fluted roll, or other-passes the seed by tube to one of several furrow openers, which are forced into the soil by springs or weights, with a short length of chain dragged behind to cover the seed. Drill widths are determined by the number and spacing of furrow openers.