The Disc Harrow: Dished Wheels Set at an Angle

A plough turns the soil over in slabs; a seed needs it broken fine. Tine harrows dragged through clods catch and choke on trash. A disc harrow rolls through instead: rows of dished steel discs on shafts turned at an angle to the direction of travel, so each disc both rolls and scrapes sideways, cutting clods and residue and throwing the soil aside. Silas A. Moody of San Francisco patented a "sod-cutter" of sharp circular knives on an axle in 1866, and Frederick Nishwitz of Brooklyn patented a revolving harrow in 1869 with dished discs under two beams set in a V, their angles adjustable — generally credited as the forerunner of the modern disc harrow. This rung splits a disc's motion into rolling and scraping, shows why gangs come in opposed pairs, and measures draft and side force on a bench gang in sand.