return; most do this twice at the fire end and once at the smokebox end, so that the steam travels a distance of four times the header's length while being heated. The superheated steam, at the end of its journey through the elements, passes into a separate compartment of the superheater header and then to the cylinders as normal. Damper and snifting valve The steam passing through the superheater elements cools their metal and prevents them from melting, but when the throttle closes this cooling effect is absent, and thus a damper closes in the smokebox to cut off the flow through the flues and prevent them being damaged. Some locomotives (particularly on the London and North Eastern Railway) were fitted with snifting valves which admitted air to the superheater when the locomotive was coasting. This kept the superheater elements cool and the cylinders warm. The snifting valve can be seen behind the chimney on many LNER locomotives. Front-end throttle A superheater increases the distance between the throttle and the cylinders in the steam circuit and thus reduces the immediacy of throttle action. To counteract this, some later steam locomotives were fitted with a front-end throttle placed in the smokebox after the superheater. Such locomotives can sometimes be identified by an external throttle rod that stretches the whole length of the boiler, with a crank on the outside of the smokebox. This arrangement also allows superheated steam to be used for auxiliary appliances, such as the dynamo and air pumps. Another benefit of the front-end throttle is that superheated steam is immediately available. With a dome throttle, it took quite some time before the super heater actually provided benefits in efficiency. Cylinder valves Locomotives with superheaters are usually fitted with piston valves or poppet valves. This is because it is difficult to keep a slide valve properly lubricated at high temperature. Applications The first practical superheater was developed in Germany by Wilhelm Schmidt during the 1880s and 1890s. The first superheated locomotive Prussian S 4 series, with an early form of superheater, was built in 1898, and produced in series from 1902. The benefits of the invention were demonstrated in the UK by the Great Western Railway (GWR) in 1906. The GWR Chief Mechanical Engineer, G. J. Churchward believed, however, that the Schmidt type could be bettered, and design and testing of an indigenous Swindon type was undertaken, culminating in