correctly required experience and depended on the speed of the train. The Roscoe lubricator was invented by James Roscoe in 1862 and patented in British Patent 1337. It incorporated two improvements over the original Ramsbottom type, the ability to control the quantity of steam which entered the lubricator, via a control valve, and the addition of an air filled chamber within the oil reservoir. The chamber expanded when steam was cut off, the aim of which was to supply oil even when the locomotive was coasting. However, this proved ineffective in practice, particularly if the locomotive was coasting for a long time (e.g. down a hill). Replacement When more sophisticated lubricators, such as the Wakefield and Detroit types, were developed, displacement lubricators fell out of use but they are still used on model steam engines. Hydrostatic lubricator The one-piece lubricator body is a gunmetal casting and incorporates the condensing chamber, the oil reservoir and the sight-glasses fitted at the front and back of the body. Passages within the body interconnect these areas. The body is fitted with a steam valve, a water valve and oil control valves. In operation, steam enters the condensing chamber and the condensed water passes into the bottom of the oil reservoir and the sight-glass chambers. When the sight-glass chambers have filled with water the oil delivery for each feed, regulated by control valves, is released upwards into the water-filled sight-glass chambers. The rate of delivery, typically two or three drops of oil through each feed per minute, is monitored by watching the progress of the drops as they rise through the water. The oil gathers above the water and passes into the delivery tubes. The body is fitted with a drain for emptying the condensed water from the oil chamber before it is refilled with oil. This type of hydrostatic displacement lubricator was made by the Detroit Lubricator Company of the USA. A British patent was granted in 1911 and this lubricator was then manufactured by the Vacuum Oil Company (later Mobil Oil) as the British Detroit Lubricator. This lubricator was commonly made with either 3,4 or 5 feeds, the center feed on the odd numbered lubricators was often used for supplying oil to the steam end of Westinghouse air compressors. Wakefield lubricator The Wakefield Oil Company, a manufacturer of lubricating oils, was founded in London by Charles Wakefield in 1899. The company manufactured both