Locomotive boiler water feed apparatus. Water in the boiler is consumed as steam is generated and it is essential that the water is replaced quickly to allow steam production to be maintained and to prevent too low a water level causing a collapse of the firebox crown. Early locomotives were equipped with mechanical pumps operated by hand or driven off the valve gear or eccentrics. Of course, these were only operational while the locomotive was moving and it became the practice to top up boilers of stationary locomotives by positioning the locomotive against a set of buffer stops, greasing the rails under the driving wheels and applying steam to drive the wheels. This got the water pump working and allowed the boiler to be replenished without moving the locomotive. In 1858 a French engineer named Henri Giffard invented the injector, a steam powered system for replenishing locomotive boilers. In the US, Messrs William Sellers of Philadelphia started selling them in 1860, the first being applied to a Baldwin locomotive. Early versions of injectors used live steam forced through a series of cones whilst mixed with water from the tender. The pressure of the steam forced the water into the boiler. The application of steam to the injector was controlled by a cock in the cab. Later versions of injectors used exhaust steam piped from the cylinder exhaust while the engine was under power but used live steam at other times. The changeover was automatic. This system saved steam (and therefore running costs) and eventually became common around the world. The principle of the injector is based on the fact that steam escaping from a nozzle has a greater velocity than that of a jet of water issuing under the same pressure from a boiler. If cold water is added to the jet of steam, it begins to condense and the velocity of the steam will increase sufficiently to overcome the pressure of water in the boiler. By this means, water can be introduced into a boiler against its internal pressure. Some injectors used a combination of exhaust steam and live steam. A connection at the base of the blast pipe was run to the exhaust part of the injector where it heated the feed water before it passes to an auxiliary injector. The auxiliary injector used live steam to force the water to the boiler. This type was patented by JJC and RD Metcalfe in 1908 and was claimed to save up to 15% on fuel and water. There was a type of injector, with features patented by J Gresham in 1884 and 1887, which was a "vertical restarting injector". Steam supply and feed water passed through the flange by which it was attached to the boiler. There was also a Davies and Metcalfe type patented in 1899 and 1907 which was designed to operate with feed water too hot for an ordinary injector. Injectors are tricky instruments and require a degree of skill to "prime" them and get them working. This is normally the task of the fireman. Once the steam is turn on, the right balance of water being applied has to be found. This will only work if the steam and the water are at the correct pressure. A balance also has to be found between too little and too much water being in the boiler. Too little risks melting the fusible plug, too much risks boiler water rising to reach the regulator, known as "priming", and getting into the steam pipe leading to the cylinders.