of water. Where coals are plenty and cheap, steam may be advantageously used for working the machine." Steam-powered machines used belts connected to a traction engine; often both engine and thresher belonged to a contractor who toured the farms of a district. Steam remained a viable commercial option until the early post-WWII years. Modern developments In Europe and Americas Modern day combine harvesters (or simply combines) operate on the same principles and use the same components as the original threshing machines built in the 19th century. Combines also perform the reaping operation at the same time. The name combine is derived from the fact that the two steps are combined in a single machine. Also, most modern combines are self-powered (usually by a diesel engine) and self-propelled, although tractor powered, pull type combines models were offered by John Deere and Case International into the 1990s. Today, as in the 19th century, the threshing begins with a cylinder and concave. The cylinder has sharp serrated bars, and rotates at high speed (about 500 RPM), so that the bars beat against the entire plant as it is mechanically fed from the reaping equipment at the front of the combine to the gap between the concave and the rotating beater/cylinder. The concave is curved to match the curve of the cylinder, and the grain, now separated from the plant stalks falls immediately through grated openings in the concave as it is beaten. The motion of the rotating cylinder thrusts the remaining straw and chaff toward the rear of the machine. Whilst the majority of the grain falls through the concave, the straw is carried by a set of "walkers" to the rear of the machine, allowing any grain and chaff still in the straw to fall below. Below the straw walkers, a fan blows a stream of air across the grain, removing dust and small bits of crushed plant material out of the back of the combine. The residues fall to the ground and occasionally are collected for other purposes, such as fodder. The grain, either coming through the concave or the walkers, meets a set of sieves mounted on an assembly called a shoe, which is shaken mechanically. The top sieve has larger openings, and serves to remove large pieces of chaff from the grain. The lower sieve separates clean grain, which falls through, from incompletely threshed pieces. The incompletely threshed grain