'dwt') is a measure of a ship's carrying capacity, including bunker oil, fresh water, ballast water, crew and provisions. It is expressed in tonnes (1,000 kg) or long tons (). This measurement is also used in the U.S. tonnage of naval ships. Increasingly, tonnes are being used rather than long tons in measuring the displacement of ships. Harbour ton, used in South Africa in the 20th century, was equivalent to 2,000 pounds or one short ton. Assay ton (abbreviation 'AT') is not a unit of measurement but a standard quantity used in assaying ores of precious metals. A short assay ton is grams while a long assay ton is gram. These amounts bear the same ratio to a milligram as a short or long ton bears to a troy ounce. Therefore, the number of milligrams of a particular metal found in a sample weighing one assay ton gives the number of troy ounces of metal contained in a ton of ore. In documents that predate 1960 the word ton is sometimes spelled tonne, but in more recent documents tonne refers exclusively to the metric ton. In nuclear power plants tHM and MTHM mean tonnes of heavy metals, and MTU means tonnes of uranium. In the steel industry, the abbreviation THM means 'tons/tonnes hot metal', which refers to the amount of liquid iron or steel that is produced, particularly in the context of blast furnace production or specific consumption. A dry ton or dry tonne has the same mass value, but the material (sludge, slurries, compost, and similar mixtures in which solid material is soaked with or suspended in water) has been dried to a relatively low, consistent moisture level (dry weight). If the material is in its natural, wet state, it is called a wet ton or wet tonne. Subdivisions Both the UK definition of long ton and US definition of short ton have the same underlying basis. Each is equivalent to 20 long or short hundredweight, being and respectively. Before the twentieth century there were several definitions. Prior to the 15th century in England, the ton was 20 hundredweight, each of 108 lb, giving a ton of . In the nineteenth century in different parts of Britain, definitions of 2,240, 2,352, and 2,400 lb were used, with 2,000 lb for explosives; the legal ton was usually 2,240 lb. In the United Kingdom, Canada, Australia, and other areas that had